[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.

[storage] Added delete_transaction method

- Added storage.ledgercli implementation
- Added storage.ledgercli update_transaction
- Added storage.ledgercli get_transaction
- Pushing pre-built docs

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