Merge pull request #3 from fredrikt/master

painkiller for relative imports
Merge pull request #4 from fredrikt/master

dependency stuff
GCD no longer covered by pubkey.* import.
add some abstract methods
Remove unnecessary semicolons.
Avoid 'multiple statements on one line' pylint warnings.
Get rid of the last 'from .pubkey import *'.
Exception cleanup.
class pubkey rename to CryptoPubkey