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
I had enforced an explicit input directory a while ago because of the need to avoid jjtree configuration when using javacc alone. But this is not required anymore with the new way to detect if configuration should occurs or not which is now a check to see if we have any jjtree input files.

There is a default input directory now.