CHT: Implement IASPEI CSV format import tool

[matley]

Design and implement an API to import into the CHT database events from a file in the IASPEI CSV format.

E.g. eqcatalogue.import_events(filepath=<>, eventsource_name=<>, format=eqcatalogue.IASPEI_CSV_V1_FORMAT)

The API import operation should be idempotent, i.e. two or more import operation from the same source should not produce duplicate rows.

If a row with the same source_key is found, then the importer update the row with the latest data and warns the user about that.

Document (on the project wiki) how the mapping between fields should be done and provide acceptance tests for this feature.

The specification of the format can be found at http://www.isc.ac.uk/gtevents/search/catalogue/csvoutput/

Sample data files in this format can be generated at http://www.isc.ac.uk/gtevents/search/catalogue/