Create Transaction class that can load a yaml file
Add method to find a transaction given the slug
Rename epdq_transactions.yml to transactions.yml

This makes more sense right now, given the model
name.
Tidy up transaction loading and memoizing
Use the transaction model to lookup the slug
Add additional controller tests to ensure transaction present
Clean up calculation logic into separate calculator
Inherit exceptions from StandardError
Inherit Transaction from OpenStruct
Merge remote branch 'origin/transaction-model'