add escape/unescape per http://nodejs.org/api/querystring.html

add `escape()` and `unescape()` + minimal tests to show that they work as expected and (per the NodeJS documentation) can be overridden

This issue has been mentioned at:
https://github.com/Gozala/querystring/issues/6
https://github.com/mike-spainhower/querystring/issues/4
https://github.com/substack/node-browserify/issues/826