Allow each instance to set eco benefit currency values

Provides a model (BenefitCurrencyConversion) that stores the
eco-unit-to-currency-unit factors.

Note that these values should not actually change with localization. For
instance, if someone is the UK decides to use £ for their currency
unit, it should show as £ to everyone (or, show in a local currency but
apply the exchange rate)

If an instance doesn't specify a conversion set, currency amounts will
simply be hidden.
Add conversion factors to search hash

Since we are caching eco benefits we need to make sure that changes to
the conversion factors result in an updated hash
Merge pull request #162 from ahinz/topic/add-currency-values-to-eco

Allow each instance to set eco benefit currency values