Added background drawing method to UiContext and updated example.

It can be called in these ways:

- `uic.background().draw(gl)`
- `uic.background().color(Color::black()).draw(gl)`
- `uic.background().rgba(r, g, b, a).draw(gl)`

See [this line](https://github.com/mitchmindtree/conrod/compare/PistonDevelopers:master...master#diff-6e2d85464f4b38449263f90fbe3f5c33R169) for an example.