Reuse WP_Stream_Settings::get_active_connectors()
Include connector class checking

Before adding & registering a connector, check if the connector's class
is a subclass of WP_Stream_Connector
Introduce WP_Stream_Connectors::admin_notices()

The new method is added to the admin_notices hook by load() and will
print messages stored in $admin_notices
Fix connector labels storing
Proper `default` detection in wp_stream_filter_input

Resolves #280
Merge pull request #265 from x-team/refactor-WP_Stream_Connectors

Refactor WP_Stream_Connectors
Merge pull request #281 from x-team/issue-280

Proper `default` detection in wp_stream_filter_input