CSS Buttons Refactor

**Refactor Buttons to more bootstrappy style.**

All buttons will continue to require .btn as a root class.

Colors will be:
  - btn-primary: @brand-primary
  - btn-default: @gray3
  - btn-dark: @gray1

Widths will be: 
  - btn-xs: padding: 6px
  - btn-sm: padding: 7px
  - btn-md: padding: 10px
  - btn-lg: padding: 15px