E-Mails sent by Review Board now add a X-ReviewBoard-Diff-For header

for each file that was modified in the diff. In the case of a moved or

copied file, both the source and destination file names are added.

Otherwise only the relevant file names (source in the case of deleted

and destination in the case of new) are added.
To facilitate this change, our SpiffyEmailMessage class has been

overhauled to handle message headers. Django expects the headers to be

a plain dict, but since we are repeating headers we now use a

MultiValueDict. All E-Mail unit tests have been updated to look for

headers in the correct location and new unit tests have been added to

test the X-ReviewBoard-Diff-For header.

Unit tests pass.
Published a review request with multiple files modified. Saw multiple

X-ReviewBoard-Diff-For headers in the resulting E-Mail mesage.