Merge pull request #483 from bvssvni/master

Added `AddBorder` trait
Merge pull request #484 from bvssvni/master

Added `EllipseBorderContext` and `EllipseBorderColorContext`
Merge pull request #487 from bvssvni/master

Added more border contexts
Changed `SourceRectangle` to use `i32`

Closes https://github.com/PistonDevelopers/rust-graphics/issues/491