Market making rate & link creation

[D4nte]

To avoid issues with funds, Bobtimus needs to calculate:

• an acceptable rate
• a published rate (link)
• a maximum acceptable amounts
• a published amounts (link) Each time Bobtimus does the funding action (remember the option problem). Which means that a unique ID should be generated for a new rate each time a fund is done.

When rates/amounts are updated, a new link should be created and put it in a html file.

The published amount is min(ether_balance/i, bitcoin_balance/i). Where i is configurable or calculated on the same curve than the rates (up to implementer).

The acceptable rate is the published minus some margin to allow old links to still be valid for some times (but not forever). Similarly for the acceptable amount.

[thomaseizinger]

Done in #56.

Remaining glue to be written as part of https://github.com/coblox/bobtimus/issues/46.