Instagram integration

See #45

Adds Instagram integration. Mainly importing and fixing 3bdd301 which is not my code.

Adds a new filter `mexp_instagram_credentials` for grabbing generic Instagram credentials that will be used to query for trending pictures. Also has a `mexp_instagram_user_credentials` filter used to supply a user access token in order to fetch personalized content.

The MEXP Keyring Credentials plugin that is added allows Instagram to pull user credentials from Keyring, if it is installed and has the Instagram service activated. This plugin supports both the current released version of Keyring and the development one in use on WP.com.

Adds the ability for a page to automatically have its content fetched when it is open. This is useful for things like pulling trending content when that page is navigated to.

Closes #45