fix button text perceived vertical alignment

The text was already centered but with the box shadow and missing descenders it could appear to be top-biased.

Before:

![](http://f.cl.ly/items/2l0J2P2s0H3k1I07322H/Image%202014-10-21%20at%2010.37.54%20AM.png) ![](http://f.cl.ly/items/2S0b2n3X3d1A2s3B1h2V/Image%202014-10-21%20at%2010.38.23%20AM.png) ![](http://f.cl.ly/items/0Y0n392s1a16320X400Y/Image%202014-10-21%20at%2010.38.50%20AM.png)

After:

![](http://f.cl.ly/items/0W0L113r3Z0a0h2j1D2T/Image%202014-10-21%20at%2010.36.19%20AM.png) ![](http://f.cl.ly/items/3n0v3K0Y122j1o0w3C0B/Image%202014-10-21%20at%2010.36.52%20AM.png) ![](http://f.cl.ly/items/3t10133w1D2l3P1O0W02/Image%202014-10-21%20at%2010.37.13%20AM.png)
perhaps consider having the mixin handle the `background` too