Added new forex provider to fix "Currency Rates Source Not Ready"

[ShanePadellaro]

Added forex provider (exchangerate.host) which is almost a 1to1 replacement for the old theforexapi.com. Added a check to throw if a currency is not available since new provider always returns 200

All unit tests for converter.py are working a part from test_with_valid_currency_code which looks to be broken because the included json file doesn't contain the expected symbol.

[ShanePadellaro]

fixes #126