Add options to override GUESS_* items from .reviewboardrc

If your .reviewboardrc contains GUESS_FIELDS=True, GUESS_DESCRIPTION=True, or
GUESS_SUMMARY=True, there's no way to override these on a per-submission
basis (for example, if you want to update a review request, but don't
want to update the descipriont).

Add --no-guess-{fields,description,summary} options to rbt post, which
override these values from .reviewboardrc
Stop args from containing a single empty string

Sometimes (I haven't tracked down quite where) OptionParser will return
args as ['']. This can get interpreted as a revision range by the clients
(in particular git), which can confuse matters - the git client tries to
run `git rev-parse `, which fails, for instance.

Therefore remove emptystring from the args return from OptionParser.

Untested on any clients except git.
Merge branches 'feature/no-guess-options' and 'feature/empty-args'