feature/transactions

This is an implementation attempting to solve the #7 and #8, that is, to be able to perform the whole import process within a transaction. Tests are pretty complete, but I'd still prefer to keep this in an alpha stage within its own branch until fully tested in real environments.

For the details on how to use it, checkout the [section about transactions in the README](/continuum/active_importer/blob/feature/transactions/README.md#transactions). Also checkout the code and test this as much as possible.

/cc @michelson @glundgren