Add a next method to Step

This way we avoid duplication about final step detection from the rest
of the code
Use MicroPaymentChannel in SellerMicroPaymentChannelActor
Move deposit checks to its own class
Finally remove DefaultProtoMicroPaymentChannel
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Merge pull request #209 from sortega/issues/41

Remove ProtoMicroPaymentChannel
Add interface for PeerActor
Add replace BlockchainActor with PeerActor in ExchangeActor
Add last offer querying to MicropaymentChannel actors
Add ExchangeTransactionBroadcastActor interface
Add scaladoc to exchange actor
Add height notification capability to blockchain actor
Add constant value actor
Implement ExchangeTransactionBroadcastActor
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