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**:

| **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 chances!** (*$background-gray*, *$border-gray* and *$input-border-gray* are removed)

![billy-colors](https://cloud.githubusercontent.com/assets/2249984/3583145/6e2bd634-0c05-11e4-9f95-1b82eafa8651.png)