Configurable exchange & currency support

[colindean]

I quickly tried a s/EUR/USD/g to use USD instead, but found that the choice of exchanges didn't really return sane data for USD. Python's JSON parser takes a JSON null to a Python None, which doesn't like to be output as a float.

It'd be nice to have configurable exchanges and currency, or just pick from the exchCUR symbol from wherever.

[maxme]

Thanks I will investigate this bug and add a set in config.py to set up exchanges and currency