Added OAuth support to Gutterball's API

OAuth can be tested by sending API requests via curl. Such requests will be unsigned, so only /status should work (unless OAuth is disabled in the config).

A simple script for doing basic OAuth-signed requests is available here: https://gist.github.com/Ceiu/e7b7f33a6d0abe599ecc
It's similar to curl, except you provide the consumer and secret after the url:
./oauth_get.rb http://localhost/gutterball/reports oauth_consumer oauth_consumer_secret
I usually leave out `this` unless it is absolutely necessary.  In this line, you can get by without it.