Enh: Initial work on error handling
Fix: Added <fieldset></fieldset> for each of the sequence/delay combinations.
Enh: New filter ( pmpro_sequencepost_type ) for the post types to allow drip sequence management & metabox in the WP editor. @param $postTypes = array( 'page', 'post', ... ). @return array() - post types.
Enh: New filter ( pmpro_sequencepost_status ) to let admin decide which post types should be available to list in the metabox on the Sequence edit page. @param $postStatus = array(  'draft', 'publish', ... ) @return array() - WP post status strings.
Enh: Moved validation of delay value (from $_POST) into its own function.