Updates vars for button colors; adds buttons section to docs

Audited the color definitions in `_variables.scss`. Removed old unused color defs which were left in there from old branding. Defined button colors. Added descriptive text describing button usage and colors to documentation. 

It'll be nice to standardize mockups and comps to follow the same rules.  I added an .ase swatch file which can be downloaded from this site and used in Photoshop, Illustrator, Fireworks, etc. This file is built by the tasks/ase.php file, which I included in the repository for historical purposes. But its ugly to require yet another language for the build environment, and to require hardcoded variables. It'd be much nicer if this step could be automated, maybe as a grunt task, which would read the colors from the variables.scss file... 

Warning: THIS DOES CHANGE A COUPLE OF THE COLOR DEFINITIONS. I think they are much more sensible defaults ("success" should always be a shade of green, not the turquoise that was before; and "primary" and "secondary" for buttons now refer to orange and teal like in our branding), but we should check them against the console to be sure.