Merge pull request #451 from bvssvni/master

Added `ImageContext` and `ColorContext`
Merge pull request #453 from bvssvni/master

Implemented `AddRectangle` for `ImageContext`
Merge pull request #455 from bvssvni/master

Implemented `AddRectangle` for `ImageRectangleColorContext`
Merge pull request #457 from bvssvni/master

Deleted `AddSourceRectangle`
Merge pull request #459 from bvssvni/master

Made `AddImage` for `Context` return `ImageContext`
Merge pull request #461 from bvssvni/master

Made `AddImage` for `ColorContext` return `ImageColorContext`
Fixed image bug

Closes https://github.com/PistonDevelopers/rust-graphics/issues/462