Adds possibility of downloading PDF attached files
Avoid setting format parameter when getting a file
file method can now return raw content for more types than just PDF
Updated doc to include file method
Adds check to iterfollow to avoid unwanted exception
Adds collection method
Adds missing test PDF
Adds collection method test
Adds collection method documentation
Fixs missing comma
Merge remote-tracking branch 'mpao/fix_iterfollow' into dev
Merge remote-tracking branch 'mpao/dev' into dev

This PR incorporates the "file" method to pyzotero that will pull the raw content
of the file and return it as a binary string.
This also includes the corresponding test and documentation update.
Version bump