Change OctoKit to be an umbrella framework

As it is right now the OctoKit requires any OS X application wanting to use it to convert to an Xcode workspace and bundle ReactiveCocoa as well as Mantle. This leads to quite long and somewhat complex set up. I instead propose to bundle the required frameworks within OctoKit. This would (in my opinion) lead to OctoKit being easier to integrate. The drawback would be that it will be much harder to use a custom version of the mentioned sub-frameworks.

Also see issue #68 and pull request #69 for a different solution and [my test app/project](https://github.com/rastersize/OctoKit-TestApp) (for both OS X and iOS).