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. I included that file 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...
Remove "lead" class from <p> starting "Brand colors"

As noted in https://github.com/janrain/bootstrap/pull/21#issuecomment-59858564,
using the "lead" class here is inconsistent.
Merge pull request #19 from janrain/thanj/buttons-documentation

Updates vars for button colors; adds buttons section to docs