Gdeltdoc

Provide user agent in requests to the API (22)

Validate `timespan` filter parameter to make sure it's an allowed value
Catch API errors when a query string is invalid and return them to the user

Fix a bug in `multi_repeat` which meant any filter using `OR` would fail

Fix a bug in `multi_repeat` which caused a bad response from the API which could not be parsed

Fix bug when only the first of the filter conditions (eg. keyword, near, etc.) was used

Recursively load the JSON response to remove improper characters