Show more button final ultimate fix

This thing was driving me crazy for some time now. There is a small difference between our font and our fallback font (helvetica):
![button-text](https://cloud.githubusercontent.com/assets/2579465/5439288/8c09da04-8476-11e4-935d-d51b6ee3f9b0.gif)

This was causing inconsistencies between the browsers and more importantly it was causing a very noticeable text jump when moving between desktop and tablet breakpoints:
![mobileondesktopbefore](https://cloud.githubusercontent.com/assets/2579465/5439137/23d73630-8475-11e4-9dfe-41d0aaadddb6.png)

This PR is fixing it. However on Windows there is still a small difference in rendering.

The main magic is in increasing the font size from 13px to 14px. And using relative units for line-height.

Chrome on Mac before and now:
![chromebefore](https://cloud.githubusercontent.com/assets/2579465/5439181/91c9a97a-8475-11e4-96d7-ef3172d3be5e.png)
![chromeafter](https://cloud.githubusercontent.com/assets/2579465/5439183/945669b2-8475-11e4-96a2-b3a0ee2ce2b9.png)

FF on Mac before and now:
![ffmacbefore](https://cloud.githubusercontent.com/assets/2579465/5439187/9dca17c8-8475-11e4-90da-b5e81d90ef71.png)
![ffmacafter](https://cloud.githubusercontent.com/assets/2579465/5439188/a0809438-8475-11e4-9f0b-1145779f5c4a.png)

Tablet and less resolution on mobile with our font and with fallback:
![mobileondesktopwithfont](https://cloud.githubusercontent.com/assets/2579465/5439194/b536f6e2-8475-11e4-8b2b-1583ddf78530.png)
![mobileondesktopnofont](https://cloud.githubusercontent.com/assets/2579465/5439195/b9c7b8d6-8475-11e4-9ec1-99cf770e756d.png)

And this is how it was before:
![mobileondesktopbefore](https://cloud.githubusercontent.com/assets/2579465/5439203/c8fc2dd2-8475-11e4-8883-389f11680477.png)

IE and Chrome on Win:
![ie11after](https://cloud.githubusercontent.com/assets/2579465/5439206/de3f9c6a-8475-11e4-8356-21c301ac96c8.png)
![chromewin8after](https://cloud.githubusercontent.com/assets/2579465/5439210/e59937f0-8475-11e4-90bd-9a189fe1bd47.png)

It may be worth adding extra rule for IE.

@sndrs @sammorrisdesign @Jholder112233 @crifmulholland @Pearson82