Merge pull request #9 from cwensley/corlissdev

Fixes problems with iOS versions earlier than 6.0
Remove ProjectTypeGuids from Eto.Platform.iOS.csproj since this is a standard C# library.
Context menu key definition with a mapping for Winforms.
Allow Eto.csproj to build for mobile: conditionally compile desktop-only/mobile-only files.
iOS Build fixes.
iOS Build fixes
Add null check to avoid an exception that was encountered.
Temporary fix to test app to recognize Generator eto2.mac. (A more permanent solution is needed.)
Merge branch 'dev' of https://github.com/corliss/Eto into dev
iOS Build fixes
Re-add ProjectTypeGuids with a condition, to allow Eto.Platform.iOS to be built within Visual Studio.
Move CreateGraphics to Drawable.
Add TreeGridItem.Tag.
Added null check.
iOS WorldToScreen/ScreenToWorld.
Enable iOS UITimerHandler.
Graphics.MeasureString: Add a check for null strings and return Size.Empty. (Platforms such as iOS and Mac throw exceptions for empty strings.)
Null check.
Fix namespace.