contracts/chequebook: add chequebook contract wrapper

The wrapper code is adapted from the swarm/services/chequebook package
with the following mostly cosmetic changes:

* The code now uses the new Go API interfaces to query balances. Some
  minor functional changes were required to make this work.
* The package no longer depends on swarm/services/swap/swap. References
  to swap.Promise are replaced by interface{}, the base type of Promise.
  This is temporary.
* The contract wrapper has been regenerated with latest abigen
  and solc v0.3.6.
* There is a new generator that creates the 'deployed code' variable.
* Documentation comments now follow the recommended godoc style.
* [CHEQUEBOOK] log prefixes are gone.
* LGPL license headers have been added to all files.

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