When a review request including binary files was posted with older

versions of RBTools (or if uploading of binary files did not occur, for

example, for very large files), rbt patch would crash attempting to

access the non-existent FileAttachment.
This change makes it so we check if the attachment exists first, and

adds a variety of tests for the rbt patch command to test various

aspects of binary file patching.

Ran unit tests.