We're using ReviewBoard quite a bit now at AG Interactive and are beginning to experiment with Mercurial as our primary SCM/VCS/whatever instead of Subversion.  Initial experiments with `post-review` and pure-Mercurial repos (not hgsubversion clones) have indicated that `post-review` is geared toward use with hgsubversion clones.  This patch seeks to do the following:

    - fixes the assumption that `hg svn info` returning non-error means the working copy is an hgsubversion clone.
    - adds support for reading 'reviewboard.url' option from local hgrc (global hgrc support unimplemented)
    - adds support for posting reviews of locally-committed changes (old behavior only supported posting uncommitted changes)
    - adds support for `--parent` option to specify remote for outgoing diff
    - add tests for both hg and hg+svn client stuff

ran existing tests, wrote and ran new tests (included in patch)