Mt103

* Added support for a few more portions of the `text` section:

* instruction_code
* transaction_type_code
* exchange_rate
* sending_institution
* sender_charges
* receiver_charges

Big thanks to [Broto](https://github.com/bbroto06) again for reporting the
problem.

* Bugfix [3](https://github.com/danielquinn/mt103/issues/3): account for
application headers for outgoing messages, MUR values containing spaces, and
trailer sections containing `{}`. Thanks to [Broto Bhattacharjee](https://github.com/bbroto06)
for the report!

* Changed the nature of the `.user_header` attribute from a string to a
`UserHeader` object. This new object has the same string representation
(`str(mt103.user_header)`), but now also possesses new sub-attributes.
* Added support for user header fields including `bank_priority_code` (`bpc`),
`message_user_reference` (`mur`), `service_type_identifier` (`sti`), and
`unique_end_to_end_transaction_reference` (`uetr`).

Initial release.