Additional search fields & duplicate handling

• Add "authors", "editors" and "abstract" search fields #4
• Set copytext to Zotero URL #6
• Also recognise webpage entries #2
• Use entry ID instead of key as primary key #7

Add support for attachments

• Add support for attachments with relative paths via ATTACHMENTS_DIR setting
• Add configuration keyword zotconf
• Add self-updating
• Better handling of nameless attachments

Handle dates before 1900

v0.1.1-beta

Initial release

v0.1-beta