* previous logic for resolving boost::filesystem::path parameters in the XObjectValue class actively tried to resolve existing files
** this contradicts the planned introduction of e.g. a external "write-file" function
* callers of external functions with path arguments now have to enclose them in square brackets if include path resolution is required
** analog to the usage of the "xsl:import" tag
* moved "getPathFromSystemId" from compilation local method into static method of IncludeEntityResolver
* changed test cases accordingly

* FunctionReadFile is now able to distinguish between XML files and plain text files
** it selects the appropriate course of action automatically
** reading the file as XML into the DOM or reading it as a string
* the current selection criteria is the file extension
** I am thinking about trying to import every file into the DOM and using the result state of that action as selection criteria
* Updated README.md and test cases accordingly

* variables containing the output of a external function can be defined using the "select" attribute
** tree-like definition is only needed when helper templates such as "transformer" are used
** queries on variables that are instantiated in this fashion need the "self::" prefix before base nodes
* changed includes of the "testcase.xsl" base template to imports
** this signifies that the importing template is based on the imported template
** ... and this is exactly what we want to define

* the root node of the result tree of each function is a domain element
** i.e. the root node of "read-xml-file" is "file", the root node of "read-directory" is "directory"
* the root node contains the result state of the function call encoded in a "result" attribute
** possible values are "success" and "error"
** the root node may contain additional attributes such as the target path of a called transformation
* the actual function result is contained within the child nodes of the function root node
** i.e. the XML file tree returned by "read-xml-file" is a child of the function root node
** if specific errors occured they are also returned as child nodes of the function root node
*** this is currently only the case for "transform" where transformation errors are returned as "error" value node childs of the function root node
* updated test cases accordingly

* test case requires markdown.pl formatter to be available in "/usr/bin/"
* updated README.md to mention "external-text-formatter" instead of "execute"
* updated "read-directory" test case reference file
* included library resolution test into "read-file" and "read-xml-file" test cases

* test case base template contains the basic test case markup
** it was mostly created to include include-path resolution into the test cases
* improved transformer helper template by separating the external function call into its own variable

* the directories below "./test" contain test cases
** "transformation.xsl" and the expected result as "reference.xml"
** tests are performed by "check.sh"
*** returns diff on error
* this system enables simple verification of external function results
** way simpler than C++ unit tests would be in this scenario
* expanded cmake instructions to automatically execute the test cases
* old example transformation was removed