Implemented compileJJTree task

This is an initial implementation of a JJTree task feature. Added tests which passes and tested on my project. I will make some more tests with some JavaCC only project to ensure they are still working great.

Once we agree that it's ok with you, I will update `README.md` file.

* It is now possible to compile a JJTree file to re-use it afterward
   in when compiling JavaCC.
* Test resources have been moved in two distinct directories: one
   for `javacc` and another one for `jjtree`.

Regards,
Matt
Seems like there are 3 test cases missing:
# compileJavacc depends on compileJJTree when inputs not empty
# compileJavacc does not depend on compileJJTree when inputs are empty
# generated JavaCC files are added to the inputs of compileJavacc