non blocking exchange rate API calls

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Merge pull request #603 from wozz/exchange-fix-gui-hang

non blocking exchange rate API calls
Merge pull request #604 from CodingAnarchy/master

change estimated_fee to include 34 bytes per output instead of hard-code...
follow-up to #604
follow up to #604 - gtk gui
Merge remote-tracking branch 'upstream/master'