Preserve single quotes for values

The previous toString() call being used to generate the styles stripped
out all single and double quotes. There is a technical difference
between some quoted and non-quoted values, as discussed here
http://stackoverflow.com/questions/7638775/do-i-need-to-wrap-quotes-around-font-family-names-in-css

To fix this, the use of toString() is replaced with a very similar bit
of code that replace double quotes with single quotes. Because all
Juice'd output styles use double quotes, this is reliable.

A test case was added and one test case that had a minor failure from
the update, was updated. This should be a non-breaking change.
Merge pull request #8 from jrit/master

Preserve single quotes for values