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`
Add comments to `fiatBalance`

cc @pgte
Assert that `config.settings.lowBalanceMargin` is >= 1

Protect people from shooting themselves in the foot.
This probably matters in this proof
Merge pull request #8 from lamassu/tests

Tests