Authentication, media fixes and refactoring

relatively large release with:

Tidy: I’ve done a full tidy-up of every script to follow the Python style guide, remove redundant code and improve flexibility.
Expansive: GBIF gets full support for predicate or DOI queries, and the Flemish and Kew Gardens portals are supported for the first time. ALA gains OAuth support for occurrence queries.
Fixes: downloading media is fixed, as well as other miscellaneous bugs.