Added write requests for LastFm API. Added authorisation guide.

- Small enhancement to make it able to call write methods to LastFm. 
        a = lastfmapi.LastFmApi(API_KEY, API_SECRET)
        TOKEN = a.auth_getToken()[u'token']
        # go to URL below and confirm application authorisation
        # http://www.last.fm/api/auth/?api_key=API_KEY&token=TOKEN
        # than call & store session key

        SESS_KEY = a.auth_getSession(token=TOKEN)['session']['key']

- Fixed UTF-8 parameters values by using requests packet instead of urllib

- Added dependency for md5 generation lib - for LastFm write methods