Issue a warning when add-on is already signed. (bug 1070188)

I added a test to check if an add-on has a preexisting signature.
See: https://bugzilla.mozilla.org/show_bug.cgi?id=1070188

The test only checks the presence of signature files, not their contents.
Isn't it a bit fragile to just use lengths, I would use [sets and issubset or intersection](https://docs.python.org/2/library/stdtypes.html#set) instead.