Changing button--small line height to relative units.

We are experiencing line-height issues now on button--small. It's definitely caused by the use of mq. I was looking on the effect on other buttons but I somehow missed it.

I moved it just the button--medium. But the line-height is being calculated all over the place. I am used to use relative units for line height which are working much nicer.

This is on page now:
![screen shot 2014-10-23 at 15 58 36](https://cloud.githubusercontent.com/assets/2579465/4755512/2de5b9c2-5ac6-11e4-8d8c-f800112e2f18.png)

![screen shot 2014-10-23 at 15 58 40](https://cloud.githubusercontent.com/assets/2579465/4755517/33a4a774-5ac6-11e4-8866-1c8e82b9cdf1.png)

This will be after this fix:
![screen shot 2014-10-23 at 15 54 14](https://cloud.githubusercontent.com/assets/2579465/4755523/3f34cf92-5ac6-11e4-9b8b-f2774c850a09.png)

![screen shot 2014-10-23 at 15 53 43](https://cloud.githubusercontent.com/assets/2579465/4755525/44e70680-5ac6-11e4-8943-a11997128348.png)



FYI: I will try to refactor buttons to use relative line-height. This is just a quick patch. The relative line-height solution will have 2 major benefits.

1. more consistent calculation of button height and less magic numbers
2. less font-size: 14px; font-size X.XXrem; so less CSS.

CC @ironsidevsquincy @austinh7