Added javadoc class description.
Refactored all exceptions to its own package.
Fixed spelling (found -> fund).
Now InvestmentAccount also holds the bought investment funds.
Added toString() method and removed EffectiveDate from the constructor.
Created Fund and Stock specific transactions.
Added a method to return the price per participation.
Renamed enum to IN and OUT to avoid confusion.
Refactored fees' tests to correct package.