Yahoo finance

Hi, I implemented adapter for Yahoo finance. 
Some currency combinations probably aren't supported by yahoo, which resulted to errors in exchange/adapters/__init__.py (see diff). 

Also, Isn't it safer to to access dict this way?
usd_source = usd_rates[source]  => usd_source = usd_rates.get(source, None)

Could you check if theese changes doesn't break open exchange rates adapter?