Fixes #79 Add reverse method

Resolves #79.

I used the results of the following jsperf tests to select the reversing method; it's generally fast enough.

* http://jsperf.com/string-reverse-function-performance
* http://jsperf.com/string-reverse-function-performance-on-long-strings

I'm fine if you decide to use a different one.

I wasn't sure what behavior was appropriate for `null` and `undefined`. They currently return themselves.