git-submit has no reason not to work with master/trunk branches

Change-Id: I1001cec49a1b7055b68f5529b734ae85f01a0d02
add a url attribute to the downloaded image that specifies where it came from (might be useful later if I extend this beyond imgur stuff).

Change-Id: Iaedb5853a1818d959a60bd1da771ead453a5bc95