expanded bitcoin gui to prepare for escrow demonstration
Added a detailed example in which funds can be sent to a simulated pool and then withdrawn again when they are confirmed.
This should demonstrate how to make use of the escrow functions and what functionality is needed on the OT side.
As a clean interface to the bitcoin function I added the MTBitcoin class.
Also I started adding more comments and did various bugfixes and adjustments.

TODO: More commenting, ability to set transaction fee when withdrawing, proper user interface and misc.