Added OAuth-like signature support for API calls.

To use the signature supply the following parameters:
nonce: http://en.wikipedia.org/wiki/Cryptographic_nonce (to be stored in the database to prevent replay attacks)
timestamp: the unix timestamp when the message was created
access_token: the public access token matching the private token used to create the signature
signature: a sha1 hash of the request parameters (excluding the signature) ordered as per http://oauth.net/core/1.0/#rfc.section.9.1.1, concatenated with the private key (pseudo code: sha1(order_params(params) + private_key))