add IPv6 support; implement padding cell
implement vpadding cell
clean up crypto; remove timing attacks
Added `reason_msg` to RELAY_END cells.

This was done to get rid of a bug caused when reason=4. Also added some
better log messages.
Started using `CellNotEnoughBytesException`.
Added better log message for printing conn buf len.
fix KEY_ERROR in circuit
Merge branch 'ipv6_support' of https://github.com/nskinkel/oppy into ipv6_support