Support authenticating in RBTools with API tokens.

Review Board 2.1+ supports authenticating using API tokens. This change
adds a new --api-token= parameter for commands to make use of these when
authenticating.

Since authentication happens before accessing resources, we can't really
check for a capability before deciding whether or not to allow a user to
pass a token. Passing a token to a server that doesn't support it is
harmless, though.

Reviewed at https://reviews.reviewboard.org/r/6237/
Allow specifying svn credentials on the command-line/.reviewboardrc file.

Most people allow their svn credentials to be cached as part of the checkout,
but not everyone does. In this case, users need some way to be able to specify
them. This change adds --svn-username and --svn-password arguments (and
associated config file keys).

Testing done:
- Ran unit tests.
- Posted a change using these parameters.

Fixes bug 1556.

Reviewed at https://reviews.reviewboard.org/r/6323/