Added friends functionality

Had to fix a parsing error on the response first: oauth_request was returning HTML instead of XML.

If you initialize Goodreads with a valid access token...

request_token = OAuth::Consumer.new(Goodreads.configuration[:api_key], Goodreads.configuration[:api_secret], :site => 'http://www.goodreads.com').get_request_token
request_token.authorize_url

[Grant access with a browser and the previous URL]

access_token = request_token.get_access_token
goodreads = Goodreads.new :oauth_token => access_token

...you can now get a list of friends with...

goodreads.friends(2003928)