Add backend ability to understand shorter addresses.
Introduces addresses version 4.
Made changes suggested by nimdahk
Allow backend to send and receive version 4 addresses
Updated several missed references to version 3 addresses
log traceback on API exception
undid support for receiving v4 addresses until more chanages are implemented
Merge branch 'master' of github.com:Atheros1/PyBitmessage
Merge pull request #450 from Atheros1/master

support for encoding and decoding variable length addresses