Add Dispose() to Sprite.cs

Added Dispose() so that developers can empty out the data from
Sprite.Texture without having to call it from within.
Modify Sprite.cs for null test on Dispose()
Modify SpriteLibraryExample.csproj for proper framework

Dispose() now has null test for Texture.
Merge branch 'master' of https://github.com/alyons/Animate-Sprite

Conflicts:
	README.md
	SpriteLibrary/SpriteLibrary/Sprite.cs

Resolved Conflict via manual merge