Core improvements and JOSS submission

Several improvements to Mordecai's functioning, including better place name picking from the Elasticsearch/Geonames gazetteer, a more disaggregated Docker container structure, and numerous improvements in documentation and example usage as part of a submission to the Journal of Open Source Software. In a non-breaking API change, the service now returns an additional field containing the admin district of geocoded locations.

For more details on improvements, see the following closed PRs: #7, #12, #16