Merge branch 'remote-installs'
Plugin install endpoint: slug should never be empty

the plugin slug could have been empty if the plugin was a single file,
and not stored in a directory
Plugin Install Endpoint: use slug in the request

Instead of using the plugin id ( i.e. wordpress-seo/wp-seo ) in the request
we will use the slug ( i.e. wordpress-seo ). In browsing plugins on the .org
directory, we found that only the slug is available. I created a new function
get_plugin_id_by_slug, so that, after installation, we can convert the slug
into a plugin id, so that our newly installed plugin will pass validation
REST API: Add plugin slug to the endpoint response
REST API: Plugins: Improve get updates logic.
Merge branch 'master' of github.com:Automattic/jetpack
REST API: Site updates status. Return sites jetpack version.
Tiled Galleries: fix typo in gallery attributes
Readme: change contribute link from Trac to GitHub
Pull down translations from Glotpress.
Merge pull request #1218 from Automattic/fix-issue-1217

Plugin install endpoint: slug should never be empty
Merge branch 'master' into theme-installs