Switch to GYP (over cmake)

[springmeyer]

As discussed we need a solid cross-platform build system for the pure c++ core, but that seamlessly integrates with (or at least nests into) platform specific build systems we'll use like XCode on iOS.

Cmake should work fine, but I'd prefer to use GYP since my knowledge of GYP is greater and growing all the time via Node.js/node-gyp/node-pre-gyp usage.

[kkaefer]

Would love your input on this. The few times I have tried to use gyp I ran into basic issues (no 64 bit support). However, I'm not a fan of CMake either, the config language is very weird.

[kkaefer]

closing in favor of #21