GitLab

This removes most references to Google, with some
remaining since they may point to things which are still
relevant. References to Google Code, Google Buzz and
Google Maps have been removed

* Content in plugins/doc/Plugin_development.md from:
https://web.archive.org/web/20140821102047/http://status.net/wiki/HOWTO_Make_a_Plugin

* Point link on Plugins Admin Panel page to plugins/README.md

Rename plugins README so it doesn't conflict with plugins directory on case insensitive files systems (Mac/Windows)