[#334] refactoring in tx sending methods
[#334] save fiat currency metadata for payment requests
[#334] save payment request url with transaction
[#334] different status message for payment protocol txs
[#334] fixed (?) some race conditions when sending payments

the send dialog needs to have a chance to save metadata along with the
transaction (which must already exist in the Core Data context) when it
receives confirmation of the submitted payment request
[#450] broadcast payment request txs locally too