Decode returned JSON to PHP object

Added "stripe_decode" to config (and a blurb about what it does in the
readme) and condition applying json_decode if that config is set to
true. Set to false by default for anyone updating existing code (no
surprise breakage) and returns response with no decoding if the config
item does not exist for backwards compatibility.
Adding stripe_decode info to Codeigniter readme

…changed my mind and decided to just add this to the Codeigniter part
as well for those who may wonder if that config also applies to
Codeigniter implementations just to be on the safe side.
Merge pull request #3 from daverogers/master

json_decode stripe return (optional)