Add docs for attachments with authors

Merge pull request maknz#20 from rap2hpoutre/patch-1

Make it compatible with the new guzzle version

Merge pull request maknz#15 from iwiznia/master

Add support for attachment's title and title link

Merge pull request maknz#14 from Leenug/add_image_to_attachment

Add image_url to attachment

Fix indentation

Update README

Document new unfurling and markdown options

Tidy up config file

Update README

Fix unicode problems in JSON encoding, fixes maknz#4.

Not sure why json_encode() outright broke with a string like 日本語, it should have just returned escaped unicode like \u<something>. But telling json_encode() to not escape unicode and leave as-is seems to fix the problem so ¯\_(ツ)_/¯