Add verify-requirements util
Use verifyRequirements in script/bootstrap
Update bootstrap requires
Add better python path description
Merge remote-tracking branch 'origin/master' into cj-add-python-check
:checkered_flag: Verify python version for build

References:
https://github.com/TooTallNate/node-gyp/blob/56dcb889091a6a0594854ccc517e498b21078d5f/lib/configure.js
Merge branch 'jugglingnutcase-exec-python-for-version' into cj-add-python-check
Rename postVerification method to bootstrap
Mimic Atom’s if/else style convention
Reuse pythonExecutable variable
Update error messages
Assume the PYTHON env points to python.exe
Make all verify requirements methods take callbacks

This moves the error handling to bootstrap and also allows us to 
display a success message.
Add better python error message
:lipstick:
Use childProcess directly
Use process.exit
Reorder if/else check
Merge pull request #2457 from atom/cj-add-python-check

Create requirement verifier for script/bootstrap