Category on UIImage to return a solid color image of a specified size.

Best practices Guide says:

> * Always trail with an 'f' when sending a float literal to a `CGFloat` parameter.
> * Do not use `x.f` when there is no decimal value, instead do `x.0f`
>  * Although this compiles perfectly fine, it is unclear (especially for our clients who may not be used to this abstract notation)