WIP: revamp media lib

- Uploadcare media tab fetches all files via REST API
- Uploadcare media tab does not use Wordpress DB now
- uploaded files are added as Wordpress attachments
save one REST API request
allow uploading files via uploadcare on media lib tab, allow cropping on insert from media tab
update uploadcare-php module
update readme, move changelog from README.md
Merge pull request #6 from uploadcare/revamp-media-lib

Revamp media lib