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 thought about it a bit more and I don't think it's a good idea anymore to remove the task. I think this now because built-in gradle plugins like java does not remove a tasks when input directories are empty.

It's a better idea to keep the task and ensure it performs nothing. What do you think?