fixed typo from confusing github3.py with pygithub
skip existing labels, milestones, attachments

this is to allow merged repositories to only import identical labels
coming from multiple bitbucket repos only once
print more meaningful error message
properly handle null values in comments Json
added str to unicode conversion in comments import
skip null and empty comments when importing

we consider they are are minor changesets to speed up the import that
otherwise will timeout the connexion
disabled logs import because it's not very useful

FIXME maybe we could import them as changelog/activity .md files?