Specify Security Assumptions and Properties that Follow #229

This issue keeps track of specifying the various set of assumptions and the properties of Raiden that follow.

• If we assume that node operators behave rationally, the wormhole attack does not happen (assuming that Raiden doesn't use onion routing).
• If we assume that node operators cannot collude easily, the wormhole attack is unlikely to happen.
• If we assume the availability of the blockchain, channel participants don't lose deposits.