Remove gravatar handling from server side.

It can be done on client if needed.
Make ingest update iterate over all providers.

Create provider type registry and on ingest:update fetch all providers
and update those which have an update service.

SDUX-46
Initial implementation of aap provider

Add provider loading files from given dir and parsing them as nitf.

SDUX-46
Parse NITF subjects and keywords.

SDUX-45
Ignore old files when updating aap.

Use provider updated field and file stats to ignore files
that are older. Also set timezone for aap dates to have all
times in utc.

SDUX-45
Drop python 2.7 testing on travis and only run on 3.3