Escape quotes in map-serialize function

The behavior of quotes when interpolating variables is currently different between Ruby SASS and libsass -- until libsass matches Ruby's behavior, escaping quotes is necessary when compiling with libsass (and will also work just fine when compiling with Ruby).
Use URL encoding to serialize data in SASS

To pass data from SASS to Javascript, the map-serialize function
generated JSON which libsass fails to properly interpolate when
compiling to CSS.

Using URL encoding instead avoids this issue and can serialize key/value
pairs in SASS with more stability.

On the Javascript side, the "parseQueryString" method is added to parse
this data back into a JS object. This method is adapted from
https://github.com/sindresorhus/query-string.
Remove extra quotes added by browsers

Browsers add extra quotes to string style values - use `.slice(1, -1)` to remove them.
Merge pull request #268 from wtfribley/map-serialize-escape-quotes

Escape quotes in map-serialize function