Added Grid panel.

Taken from Moonlight and converted from C++ to C# - thanks Mono,
would've taken ages to do it myself. Doesn't quite work properly with
the nested StackPanel in the Test app, but I think that's the fault of
StackPanel.
Fixed an issue with Horizontal/VerticalAlignment

This caused the nested StackPanel in the last commit to get positioned
wrong.
Updated readme.md.