Merging with opentk/opentk

[thefiddler]

The following branch merges https://github.com/mono/opentk and https://github.com/opentk/opentk into a common codebase:

opentk/opentk#142

I will issue a PR to mono/opentk discussion over the next few days.

In the meantime, it would be awesome if you could avoid adding any further incompatible modifications to the math API. If you need to add nxm matrix math before this PR, feel free to merge any of the 2x2, 2x3, 2x4, 3x2, 3x3, 3x4, 4x2, 4x3 and 4x4 matrix implementations from opentk/opentk. They have been there since Jan2013 and they are part of the public desktop API.

Cheers.

Edit: fixed link.