Added utilities

* Adding logout button, hiding call buttons before login (wasn't being done before)
* Adding and handling config parameter to make dtmf feedback tones optional when pressing the dialpad as requested in https://github.com/opentelecoms-org/jscommunicator/issues/14
* Optional "remember me" to save display name and sip address as requested in https://github.com/opentelecoms-org/jscommunicator/issues/12
I would prefer that the config has priority over the cookie in those situations where the login box is not displayed.

Specifically:
1. if sip_uri != null use the display name from the config file if it is specified (exists, not null, length > 0)
2. use the display name from the cookie if it is available
3. use the display name from the config file if it is specified (exists, not null, length > 0)