Wiped-out old cell code; preparing for rewrite.
Drafted beginning of cell rewrite.

Almost ready to test some basic variable-length cell parsing.
Added Nik's test cell inputs.
Fixed cell naming and `cell.definitions`.

Also implemented `FixedLenCell.Header`.
Implemented required methods of `FixedLenCell`.
Misc fixes:

- Improved cell `repr`.
- Revised `payloadRange()` and `_parseHeader()` in `VarLenCell`.
- Fixed ordering bug in `test.cellInputs.commandAuthChallengeCellBytes`.
Fixed some relay cell parsing bugs.
bugfix: `RelayCell` map to subclasses is now used.
Renamed a variable for clarity.
Improved subclass selection; added encrypted cells.
Renamed `getData()` to `getBytes()`.

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

See issues #78 and #81.
fix up some __init__() issues and begin implementing AuthChallengeCell methods
Merge branch 'cells' of https://github.com/nskinkel/oppy into cells
Merge branch 'cells' of https://github.com/nskinkel/oppy into cells

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