Starting separation of model keys for various rendering engines
The first step is to highlight common structure by putting all constants in one common class.
This common class will soon be transformed into something smarter (I think)
Moved all model extraction code outside of specific template engines models.
Goals of this operation are multiple :
 - define an interface I could use for Dates hierarchies extraction
 - make code simpler to understand, as there is no more any duplication
 - made code extensible through ModelExtractors.properties file which can allow anyone to add one more model extractor simply by adding it to this configuration file (or any other loadable by JBake classloader)

This however triggers a new issue : there are now 3 or 4 files using loading mechanism defined by Cédric Champeau. Maybe it could be time to ... rationalize that a little, no ?