[Toolkit] Allow unloading assets by name.

* Add Unload(string assetName) to IContentManager.
* Move all asset path handling from ContentManager to FileSystemContentResolver so that the ContentManager is entirely ignorant of where an asset can be found.
[DirectWrite] Replace DirectWrite.Matrix by SharpDX.Matrix3x2
[SharpDX] Reduce duplicate lerp code and use MathUtil.Lerp() everywhere
[Toolkit] Fix compiling issue on WinRT
[Samples] Fix compilation with Matrix3x2
[Toolkit] Restore ContentManager's RootDirectory functionality.
(cherry picked from commit e8eb769ddfae1afe16c6ff19367ed08a6a2786cc)
[Toolkit.Graphics] Dispose internal Effect used by SpriteBatch
[SharpDX] Fix ColorBGRA.SmoothStep()
(cherry picked from commit 05f4280a61c01101678f83267501fd3c6f4ab596)
[SharpDX] Add Lerp() and SmoothStep() to Matrix3x2
(cherry picked from commit ad96d3f4c5d383ec617b4fc10fc6fb2d45603566)