Added specs for class MoneyColumn
class MoneyColumn passes specs now
Add the -@ (unary) method instead of invert
Use MoneyColumn to merge two Columns
Test whether MoneyColumns can be merged before evaluating them
Add spec for pretty_money and nationwide
Pass @options to meging columns

Fixes spec for pretty money and two columns
Remove dead comments
Made MoneyColumn a destructive method and renamed merge!

merge! will return nil when nothing has changed, i.e. the merge has
failed. This makes it more in line with ruby library methods such as
Array.flatten!
Load @csv_parser before calling learn!

Learn (learn_from) is dependent on @csv_parser
Merge pull request #29 from BlackEdder/money_column

Further improvements in nationwide csv handling