Fixing code according to the code style audit.
Referencing Xtext in the documentation.
Improve the Maven configuration for the documentation.

The Xtend and Jnario plugins are invoked during the "generate-test-
sources" phase.

The Google Maven replacer plugin is introduced for replacing macros in
the documentation with values defined in the Meven configuration.
Split the SARLParser class.

It split permits separating the checking functions related to the SARL
parsing and those related to general purpose.
Clearify the documentation, and put in the text the values from the Maven configuration.