Updated colors and shades

This updates all colors to our new color scheme. Everything is brighter and happier.
In addition, the shades are now more consistent and we no longer use special-case colors like "input-border-gray" or "background-gray".

The following should be updated in **billy-webapp** (See https://github.com/billysbilling/billy-webapp/issues/687):

| **All instances of**  | **Should be updated to** |
| ------------------------ | ------------------------------- |
| $blue-gray-dark | $blue-gray-xdark |
| $gray-xlight | $gray-xxlight |
| $gray-light | $gray-xlight |
| $background-gray | $blue-gray-xxlight |
| $border-gray | $blue-gray-xlight |
| $input-border-gray | $blue-gray-light |

**NOTE**: This has **breaking changes!** (*$background-gray*, *$border-gray* and *$input-border-gray* are removed)

![billy-colors](https://cloud.githubusercontent.com/assets/2249984/3583145/6e2bd634-0c05-11e4-9f95-1b82eafa8651.png)
I don't think these psuedo colors belong in this `$colors` list. @sebastianseilund, you agree?