Add support for default parameter values in .reviewboardrc.

There are many parameters to post-review that people would like to make
default, either through a repository's .reviewboardrc or through their
own user's. There's also some parameters people were previously able to
hard-code inside postreview.py, but that makes less sense now.

These can now be set in a .reviewboardrc. The following are accepted:
PUBLISH, OPEN_BROWSER, TARGET_GROUPS, TARGET_PEOPLE, GUESS_SUMMARY,
GUESS_DESCRIPTION, BRANCH, SUBMIT_AS, USERNAME, PASSWORD, PARENT_BRANCH,
TRACKING_BRANCH, P4_CLIENT, P4_PORT, P4_PASSWD, DEBUG, HTTP_USERNAME,
and HTTP_PASSWORD.

Tested a few of these locally.

The "master" branch value in this review request was populated by having
BRANCH="master" in .reviewboardrc.