Added api docs for KeyboardState.cs and Keyboard.cs.
Also started fixing api docs for Keys.cs.
Finished API docs for Keys.cs.
Re-worded API docs for keys.cs, keyboardstate.cs and keyboard.cs.
Merge pull request #1286 from raistlinthewiz/keyboard-docs

Added code documentation for Keys.cs, KeyboardState.cs and Keyboard.cs.
Merge pull request #12 from mono/develop3d

Merge back from develop3d