When updating an existing review request it is useful to be able to provide
a description of what has changed between versions of the diff, and why.
ReviewBoard's API supports this in the changedescription field of a
review reqeuest draft resource, and this diff enables that field to be
set via the post-review CLI.

* Tested the post-review command to update an existing review on my development server running HEAD with a comment.
* Tried to add a --change-description to a new review request (it output the error and exited)
* Ran unit test suite. 6 PASSES, 1 FAIL on the deprecated API test (because my test server is >1.5 so there is no deprecated API to test against)