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/