Include Debugger to J2V8 project

I have created initial version of [j2v8-debugger](https://github.com/AlexTrotsenko/j2v8-debugger) using _Stetho_ for communication with _Chrome DevTools_ as it was sugessted in #293.

Definitely current functionality is limited and there are some known issues.
But in general, most important features are implemented: set/remove breakpoint, step in, step out, step over and ability to see local variables is implemented.
Thus it could be already useful in debugging some complex JS issues.

This debugger could be included to J2V8 project if you find it appropriate. Otherwise it could be kept separately.
What I am not sure is licensing - degugger uses [Stetho](https://github.com/facebook/stetho) from Facebook _(for communication in Chrome DevTools protocol over web-socket)_. While j2v8 is licensed under Eclipse Public License v1.