[tests] Check transactions for errors before being added

- Added support for Exception-specific HTTP response codes for
AccountinExceptions.

[ledgercli] Versioning, error handling

- Switched to passing the command via argv instead of stdin to ledger. We
might as well as we don't use ledger's long-running mode in an effective
manner.
- Added version control of the ledger file using pygit2.A
- Added error handling in the case of an unbalanced ledger, and cruder
error handling in the case of any stderr output from ledger.
- [web] Separated transaction_get into transaction_get and
transaction_get_all.

[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.