Create requirement verifier for script/bootstrap

This moves the build verifications to a separate file. It also allows Windows users to set the PYTHON env var if Python27 is not installed in SYSTEM_DRIVE:\\Python27.

The PYTHON env var is used by [node-gpy](https://github.com/TooTallNate/node-gyp/blob/56dcb889091a6a0594854ccc517e498b21078d5f/lib/configure.js#L25) to determine the python executable location.