Added the ability to specify any url as portrait.

In addition to adding a 4th button, we also add the ability to edit the
combatant's image url from the dropdown. This allows combatants to be
created, and images added for them after the fact, without having to
re-make the combatant.
rebased; changed to new dropbox url.
Merge pull request #63 from lordnull/feature-cc-combatant-image-url

Added the ability to specify any url as portrait.