Adding global WP.com account. Refs #496

 * Credentials are stored in Preferences and uses the existing
encryption methods for the password.
 * Incremented db version and added migration script that attempt to
migrate the WP.com credentials from the first matching WP.com blog.
 * Added basic GCM setup. App ID needs to be added to the
config.properties in res/raw as gcm.id
 * Added WordPress.com section to preferences, and added capability for
!AddAccountActivity to be used as a WordPress.com authenticator of
sorts.