Removes references to Gizmo (R.I.P), adds CancelCall and getVoicemailMP3 functions.

Removes references to Gizmo (killed by Google - R.I.P.)
Adds cancelCall function to cancel an in-progress call
Adds getVoicemailMP3 function to get MP3 file of voicemail
Cancel in-progress calls, Download unread voicemails as MP3 files.

Cancel in-progress calls previously placed using callNumber function.
Download all unread Google Voice voicemails as indididual MP3 files.
Included added functions in the description.

Added language indicating it is possible to:
- cancel previously placed calls,
- add a note to or remove a note from a message or voicemail,
- mark a message or voicemail as read or unread,
- download MP3 files of voicemail.
Merge pull request #9 from Cam-/master

Removes references to Gizmo, adds CancelCall and getVoicemailMP3 functions