[test] Added unit tests

- Moved the TransactionNotFound exception to a more appropriate place.
- Changed the serialization for AccountingExceptions
- Override the Exception.__init__ method in AccountingException
- Added __eq__ methods to accounting.models.*
- Catch the TransactionNotFound exception in transaction_get and return a
404 instead. This could be improved, perhaps in the jsonify_exceptions
decorator so that all endpoints that raise a TransactionNotFound
exception automatically return a 404.

SQL, GTK

- Made the storage model slightly more flexible
- Made a small P-o-C GUI application in GTK
- Polished accounting.client
- models.Transaction.id is now a str
- Fixed transaction.id marshalling for storage.ledgercli

Added SQL storage backend

- Added Storage ABC
- Moved Ledger class to storage/ledgercli.py
- Added SQL requirements to requirements.txt

Added client, other fixes

- Switched from int to decimal.Decimal
- Moved models to accounting.models