E.g. If a cloud provider offers SIP Gateways to the PSTN, this new
feature allows the B2BUA to use accounts on this external SIP server to
bridge calls to telephone numbers.

This bridge does not attempt to hide itself by overriding the From
header.

---

The B2BUA server now runs a CLI socket listener as well, and the CLI
framework as been improved to allow generic register & dispatch capability.
Any class can implement the CliHandler interface and register
itself to a CommandLineInterface instance to handle commands.

This new bridge responds to the SIP_BRIDGE INFO command.

External accounts used for bridging are configured via a JSON file
(see configuration)

Jsoncpp is now a required dependency of the B2BUA and an option has been
added to build a vendored version when the system doesn't provide an
adequate one.