VoIP Plivo module: merge changes from 2012-02-17

updating global $user based on voipcall uid
Rename hook_getvoices() to hook_voip_getvoices()
Added 4th param to VoipVoice::getVoices()
Delete voip_default_server when uninstalling.

Added changes so that each server is going to keep track of its
current voice, default voice, default man voice, and default woman
voice. We don't want to have a situation where we change the server
and the global variables are still pointing to voices that are not
implemented by the new server.

Removed static methods from VoipServer:
getVoicesList()
getDefaultVoice()
getVoiceId()

Changed 4th param of getVoices()
Moved setDefaultVoice(), setDefaultManVoice(), setDefaultWomanVoice(), getDefaultVoice(), getDefaultManVoice(), getDefaultWomanVoice() to VoipServer.

Fixed bug with VoipScript::setVoice().
fixed a bug associated with the direction of sms / text messages sent by twilio and plivo