Added the ability to write to the Depends On field for a review

through the rbt post tool. Through the use of the flag

--depends-on, one can specify review numbers (comma separated)

that the review depends on. The new flag has been added to the rbt

post manual.

The manual was tested by using 'make html' to ensure that the

formatting was correct. For testing the flag, I created two fake

reviews (id: 1 and 2) on my local machine. Then I made, added, and

committed a change to the reviewboard repository. I ran the post

command with the flag --depends-on 1 and checked to ensure it added

it in the web view. I then tested with --depends-on "1, 2" to ensure

it supports multiple revisions. Finally, I tested --depends-on "test"

to ensure it was checking arguments.