Ported Ios7 colors.

Ported from https://github.com/mobilemagicdevelopers/UIColor-iOS7Colors, fixing a couple of bugs and reusing `UIColors+Hex`.

![ios7colors](https://cloud.githubusercontent.com/assets/542335/2783933/2adda48c-cb34-11e3-831b-f134774f35d3.gif)

Closes #16.

@thisandagain, take a look