We had a few spots where we constructed a RBClient and fetching the

API root, each time passing in a string for the user agent and for the

cookie path, with TODO comments saying we should consolidate this

information.
This change does just that. It introduces rbtools.utils.api, which

provides get_api_root(). This takes in a server URL and credentials,

and returns the root resource.
That handles RBClient construction, passing in a user agent (now set

to ReviewBot/<version>, rather than just ReviewBot) and a cookies

file.
The cookies file still needs to be made configurable, but that will be

handled separately.
Unit tests have not been added for this new function, since it's pretty

well covered by existing tests (both in Review Bot and RBTools), and

there's not much happening beyond passing in arguments and called

pre-tested methods.

Unit tests pass on Python 2.7 and 3.x.