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.
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 'pr/6'
Merge branch 'pr/5'