Make indicator handler check for source.
Make IP addition check for source.
Make PCAP handler check for source.
Make raw_data handler check for source.
Fix bug in previous commit.
Make Sample handler catch no source.
Set tag_list to an empty list by default.

This was triggered when adding a screenshot through the API with no
tags. It would throw an exception because tag_list was passed in as None
here.
Update documentation to note "description" on screenshot.
Fix target handler when called from API.

This handler assumed that data was always fully populated, which is not
always the case when using the API. Fix it by checking for the existence
of a key before setting the attribute.