Remove activesupport dependency

This library is only using Active Support in two places, so I thought it would be better to remove it:

1. Symbolization of keys
 * This is implemented as an extension of `Hash`.
2. Calculating the length of multibyte strings.
 * This can be done using String#unpack("U*")

I've also include a few other clean-up commits, which you can cherry-pick around if you prefer.

`ActiveSupport::JSON` was also being used in the tests but that can be replaced with `JSON`, which is part of the standard library as of Ruby 1.9. I added the `json` gem as a development dependency for the off chance someone is doing development in Ruby 1.8.

Tests are passing on MRI 1.9.3-p194.