This makes the user agent string used for clients accessible from RBClient

as a property. We also weren't passing on the user agent from SyncTransport

to the ReviewBoardServer, so if a custom agent was set on the

SyncTransport, the ReviewBoardServer would still use the default agent.

This change fixes that.

Ran unit tests.
Used in an upcoming change where I access the user agent from the

RBClient in a command.