Issue-20: Added inital trimmed down version of breakpoint mixin

I trimmed down the original a bit already, but still left most of the features. This will be a good starting point to trim down from if we want something that's a bit smart, if we want something very simple, we'd be better off starting from scratch.

Features (that could be cut to reduce size/complexity):
* Breakpoints are stored in a list var with human friendly names
* Can define query to be breakpoint and up, and down, or just the specified breakpoint (e.g. `'small', 'and up'`, or `'large', 'and down'`, or just `'large'`)
* Can define what pixel density you'd like
* Error reporting is breakpoint doesn't exist
* Breakpoint debug mixin that will add a short tag to the top left corner to tell you which breakpoint you're currently viewing

**Original's documentation**
http://codepen.io/wesruv/full/eIpuv/
'-minor' and '-major' breakpoints were removed.

**Original Issue**
https://github.com/Lullabot/windup/issues/20