Added support for underline of the TextField.

I've added support for underlining the textfield.

There is a new bool value `hasUnderline` which defaults to NO. If you turn it on the - a straight line at the bottom of the TextField will appear. 

You can customize the `underlineColor` and `underlineHight` of the line.
The default values are `underlineColor = 0.5f` and  `underlineHight = [UIColor lightGrayColor]` which draws a perfect 1px line on retina displays.

Also added the corresponding tests.

I've used your component in my project and just wanted to share the enhancement I had to make for the given design. Major thanks for building this - it really is a beauty!:)