[DO NOT MERGE] Swap two steps

[krzysztof-cywinski]

This PR implements SwapTwoSteps exactly as defined in the Bacen documentation.

Assumptions and inner workings:

• when a swap is initiated the balance required to complete the swap is frozen both in the senders RealTokenizado and in RealDigital
• the balance is unfrozen just before swap execution or when the swap is cancelled
• only the recipient can execute an initiated swap
• both the sender and the recipient can cancel the swap if it's not expired
• anyone can cancel a swap that has expired

This branch is built on top of the SwapOneStep branch. Merge PR https://github.com/ioralabs/realdigital_smartcontracts/pull/14 before reviewing this one.