Unref polling timers

This makes event loop die even with those timers going. It shouldn't be
a problem since we have a server running anyway.

This is needed for tests which would just hang open otherwise.
Start a test for `fiatBalance` function
Add more tests for `fiatBalance`
Restore `fiatBalance` to working state
Add test for rate correctness, with a Bitpay mock
Add support for mocking Blockchain
Depend on development version of `hock`
Merge branch 'tests' into real-exchanges