Mach debugger flag

Borrows two python modules from mozilla-central to give mach the ability to detect and pass arguments to a system's preferred debugger, and adds `--debug` and `--debugger` flags to `./mach run`. This works almost like the functionality described [here](https://developer.mozilla.org/en-US/docs/Debugging_Mozilla_with_gdb), but at the moment it lacks a `--debugparams`  argument.

Links to borrowed files:
http://hg.mozilla.org/mozilla-central/file/c9cfa9b91dea/testing/mozbase/mozinfo/mozinfo/mozinfo.py
http://hg.mozilla.org/mozilla-central/file/c9cfa9b91dea/testing/mozbase/mozdebug/mozdebug/mozdebug.py