Ethereum payment channels for Python

This is an improved implementation of the https://github.com/obscuren/whisper-payment-channel contract.

Changelog:

• renamed owner and beneficiary to sender and receiver, validUntil to expiry
• to avoid sender using one channel for different receivers, added an address field to the payment channel data structure
• included the channel contracts address in getHash, to prevent reuse of signatures in a new contract instance
• only allow receiver to claim channel, to prevent attacks resulting in costs for opening and closing new channels

Open questions

Should the contract use the receiver address or channel id as event index? (except for NewChannel)

Is there a reason to use hash as channel id instead of incrementing integer?