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

This changes needs to be merged to D7:
updating global $user based on voipcall uid
Rename hook_getvoices() to hook_voip_getvoices()
Tropo readme changed.
Set send_text_status variable in Tropo.
Added 4th param to VoipVoice::getVoices()
Delete voip_default_server when uninstalling.
Tropo README patch
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 in Tropo which didn't allow for variable number of input digits.
Fixed bug with wrong destination number in addSendText() command.
Fix for Handling spaces and special characters in audio URLs