Began adapting old `cells` work.
`OnionRelayCell.__init__()` implemented.
create netinfo payload
Merge branch 'Connection' of https://github.com/nskinkel/309Project into Connection
wrote netinfo cell
fixed minor errors in NetInfoCell; started NetInfoCell testing
fix typo in NetInfoCell.get_src_addrs()
Implemented `VersionsCell` and `AuthChallengeCell`.
Merge branch 'Connection' of https://github.com/nskinkel/309Project into Connection

Conflicts:
	oppy/circuit/cell.py
wrote NetInfoCell tests
Merge branch 'Connection' of https://github.com/nskinkel/309Project into Connection
fix dumb error from premature merge
fix NetInfoCell syntax errors
Added initial `VersionsCell` unit tests.
fixed guido's misguided design choices
fixed some test problems
Added `payload` opt args for building from bytes.
Merge branch 'Connection' of https://github.com/nskinkel/309Project into Connection

Conflicts:
	oppy/tests/test_cell.py
Added `RelayBeginCell` and some `__repr__()`.
Made the `recognized` argument more explicit.