[Serusers] uac module

[Elena Ramona Modroiu]

I have just committed a new module "uac" that implements basic SIP UAC
functionalities like:
- manipulation of From header URI (caller id blocking, anonymization)
- client-side authentication (SER should be able to authenticate itself
  as response to a challenge reply)
- hopefully more in the future :-) .

This first version has a known limitation in authentication part - qop
is not supported. Any feedback regarding the functionality or possible
errors is highly appreciated.

Ramona