Added "--username=USERNAME" command line option to post-review to allow specifying the login username.

This works in conjunction with "--submit-as" for submitting a review as another reviewer without needing to know their password.  We use a post-commit subversion hook script that submits review requests automatically, but the "--submit-as" function required submitting the password for the submit-as user.  --username allows an "svn" user to submit review requests automatically without needing to know each submitter's password.

Tested with local Review Board instances to make sure that username and submit-as can be supplied simultaneously.