Integrate php client

This integrates the new PHP API client into our Wordpress plugin

**Notes:**
- It uses composer to load package dependencies (the client repo is private for now, until properly vetted)
- Commits of note are: https://github.com/mediacore/mediacore-wordpress/commit/be99507b12a2ac84bcd3ee0b997c8afb154f1e79 and https://github.com/mediacore/mediacore-wordpress/commit/294eb475e0afc1802582e283f68c906c3851c634