Renamed `getData()` to `getBytes()`.

All calls to `getBytes()` dynamically concatenate the appropriate
fields.

See issues #78 and #81.
Merge branch 'cells' of https://github.com/nskinkel/oppy into cells

Conflicts:
	oppy/cell/cell.py
	oppy/cell/relay/relay.py
Fixed various bugs in various `getBytes()` code.
Merge branch 'cells' of https://github.com/nskinkel/oppy into cells
Quick fixes to `AuthChallengeCell()`.