added thumbnail media type

"For image-based updates, picture and thumbnail parameters are both required." https://bufferapp.com/developers/api/updates#updatescreate
Update README.md

changed "image" media type to "picture" and added required "thumbnail" for pictures per https://bufferapp.com/developers/api/updates#updatescreate
Update README.md
added update 'attachment' option

https://bufferapp.com/developers/api/updates#updatescreate
added createUpdate 'attachment' option

https://bufferapp.com/developers/api/updates#updatescreate
Update Client.php
added createUpdate 'attachment' option

https://bufferapp.com/developers/api/updates#updatescreate
Update Update.php
boolean values in createUpdate need to be strings, as boolean false leads to default being used. per issue #2
Merge pull request #1 from dhjw/master

Was getting errors updating with a picture without a thumbnail, added attachment option, updated setting of boolean vars in createUpdate