Implement Windows RT platform

[majorsilence]

It would be nice at some point if this project implemented windows rt back end. This would be similar to the iOS (#3) and Android (#4) back ends.

[cwensley]

Agreed, this is a very important platform going forward. I haven't looked much at windows RT yet, but hopefully at least some of the code from the WPF platform can be shared.

[corliss]

I've made some major strides in the last week on this, although much still remains.
The code for the new platform, Eto.Platform.Xaml, is in https://github.com/corliss/Eto/tree/dev3, which is my working development branch, that is always synced with picoe/develop.

The changes include a Portable Class Library version of Eto, as well as a WinRT solution that includes a Windows Store test project. The platform uses Direct2D/DirectWrite as the drawing toolkit and Xaml as the UI toolkit. Be warned, there is a lot that's broken, so if you'd rather wait until things stabilize, that's fine. On the other hand if you're interested in helping, and have a good bit of experience with Eto innards, we can collaborate on this branch.

Curtis is somewhat busy these days, and plans to release 1.3 before pulling this branch. In the meantime my goal is to have as much of this platform running as possible.