pass a few more arguments down to sass

I've added `unixNewlines`, `precision`, and `cacheLocation`

`precision` is especially important for folks who use twitter
bootstrap as precision needs to be set to 10, but the default
is 5.
Replace quickTemp with broccoli-writer

Inspired by this change:
https://github.com/joliss/broccoli-sass/commit/a1c305f473e631af1b60ef4925f8e55d25910e15
add README file

Obviously most of the work for this was done by @joliss, so she gets the
credit.

I think the main part missing from this is how to use the options that
were added to ease interaction with ruby, such as bundleExec, loadPath,
and require
Merge branch 'add-readme' into combined-changes
Merge branch 'use-broccoli-writer' into combined-changes