Implemented new "layered" site generation architecture
* "source" directory contains layers as subdirectories
** ordered by their name
** e.g. layer 0 is "00_content" and contains the content alongside some metadata
* transformations contained within the "source" layers are processed sequentally
* transformations define their requirements in a "meta" variable
** the "meta" variable is interpreted by the core transformation "generate.xsl"
* requirements are currently datasources and target information
** every transformation may have one datasource of type "main"
*** this data source offers e.g. the option to iterate over it
** every transformation may have a arbitrary number of "support" datasources
*** e.g. "meta" is a support datasource
** the target node may provide a fixed target path or a Xpath to be evaluated
* the result of each transformation is written to the appropriate layer of the "result" directory
* this approach to XSLT based static site generation should be quite flexible and offer good expandability
** e.g. adding new datasource options and types