Raise ValuerError when Exchange rate is not defined, instead of returning None when converting Money between Currencies.
Raise a specialized exception ExchangeError when converting between currencies without an exchange rate specified.
Merge branch 'master' of https://github.com/Floqq/Rockefeller

Conflicts:
	rockefeller/money.py
Minor style fix.
Merge pull request #1 from robertjerovsek/master

Minor update to logic when working with exchanging money.

Raising ExchangeError whe exchanging money between currencies and the exchange rate cannot be found.