Require calculator class in exchanger file
Fix: calculator should be an instance, not a class
Make calculator exceptions more informative
Adjusting acceptance tests

Since CurrencyExchanger and Calculator are implemented we can move on with the
acceptance tests. find_account and find_rate are fleshed out as well Account and
ExchangeRate receive basic interface.
Spec and implementation of Account

The interface of Account is well specified - we need balance, withdraw and
deposit. So we can write down this expectations in spec and write the
implementation accordingly.
More specs and docs for Account

Account class has more specs (for unusual and invalid conditions) and it also
has a documentation. Some method parameters were changed in order.
Adjust acceptance tests to Account class

The Account class was implemented so we have to fix some minor issues in the
acceptance tests. The acceptance tests pass and we are happy, since new feature
was delivered to our system.
Add Rakefile for tests

A simple Rakefile was provided for running all specs and acceptance tests at
onece.
Type

  rake test:spec

to run specs. Type

  rake -T

to see the other options.