Cleaning up postreview.py

This commit creates a new python package called 'reviewboard'.  

* The ReviewBoardServer class has been moved to to reviewboard/reviewboard_server.
* All the utility classes that extended urllib2 classes are now in reviewboard/custom_http.
* The code that put together all of the command line options is now in reviewboard/option_parser_factory.
* The raw options object has been replaced with a new class 'PostReviewOptions' in reviewboard/post_review_options.  This class has some methods that seemed appropriate to place here.  It delegates option retrieval to the options object populated by the option parser.
* postreview.py has been slimmed down.  I've broken out meaningful functions where I could.  It's about 300 lines now.
* Similar to my last pull request, HTTP auth headers are being sent on every request.