Bug 867817 - Add mach core to simplify running emulator unittests

Currently running mochitests/reftests/xpcshell is very hard and confusing. I've written mach commands which reduce these tasks down to './mach mochitest-remote' etc.

This pull request is 95% just copying mach core from m-c. The two files of interest are 'mach' and 'python/mach_b2g_bootstrap.py'.