Don't break when a commit's committer is not on github
Don't break when a commit's committer is not on github

When the author or commiter of a commit are not known to github,
RepoCommit.__init__ used to crash.

Examples of such commits are most commits in edmonds/nss_ubdns

wbrefvem/github3.py,christophelec/github3.py,icio/github3.py,jim-minter/github3.py,degustaf/github3.py,krxsky/github3.py,h4ck3rm1k3/github3.py,ueg1990/github3.py,itsmemattchung/github3.py,agamdua/github3.py,balloob/github3.py,sigmavirus24/github3.py