updated __all__ in stix.bindings.stix_common

Doing "from stix.bindings.stix_common import *" fails due to
nonexistent ContributorsType. Fixed this problem by updating the __all_
list with the current set of Types
Merge pull request #167 from chongkim/stix_common_export

updated __all__ in stix.bindings.stix_common
Merge remote-tracking branch 'origin/master' into vocabs