Bug 794506 - Part 1: Move virtualenv code under mozbuild; r=ted

--HG--
rename : build/virtualenv/packages.txt => build/virtualenv_packages.txt
rename : build/virtualenv/populate_virtualenv.py => python/mozbuild/mozbuild/virtualenv.py
Bug 794506 - Part 2: Add virtualenv APIs; r=ted
Bug 917988 - Sphinx managed build system documentation; add mach build-docs; r=bsmedberg

--HG--
rename : python/mozbuild/dumbmake/README.rst => build/docs/mozbuild/dumbmake.rst
rename : python/mozbuild/mozbuild/frontend/README.rst => build/docs/mozbuild/frontend.rst
rename : python/mozbuild/README.rst => build/docs/mozbuild/index.rst