Drafted `RelayBeginCell`.
Drafted code for various relay cells.

Includes RELAY_BEGIN, RELAY_CONNECTED, RELAY_DATA, RELAY_DROP, and RELAY_END.
Merge branch 'cells' of https://github.com/nskinkel/oppy into cells
Drafted `RelayExtend2Cell`.

Also:

- Added comments indicating that implementations of extend and extended
  cells can be ignored for now.
- Remove unused code from `oppy.cell.util`
Bug-fixes in `RelayExtend2Cell` and `RelayEndCell`.