Refactored access modifiers for variables
Add currency codes

To be able to show the currency symbol of
the currency that the user has chosen the Android
currency class needs a currency code to get a symbol.
Later on this might be refactored using Unicode to
show all of the world currency symbols.

-refactor the name of the currency_symbols to currency_codes
-put the currency codes into the currency_codes txt file
-add currencyCode field to my own defined Currency class
Add BudgetPlanner implementation plan file

I added an implementation plan file for BudgetPlanner
that has the approximate plan and schedule for
implementing the app. The file also has the pictures
of how the app should like once finished and features
that should be added in future versions of the app.

- add file of the implementation plan and schedule