#215 Provide bindings for non-RPi development

[Sam]

When developing code, I don't always do it on my raspberry pi - often choosing more powerful or portable devices. When writing code that uses this library, I can't currently 'pip install' RPi-GPIO on non-Raspberry Pi hardware, for obvious reasons.

But it would be great if pip install could still pull down a different version of the library which contains just the bindings/method signatures and docs to keep muy IDE happy and make development a breeze!