Enh: Cache timeout for sequence post list (refresh).
Fix: No options loaded if PMProSequence class was loaded w/o a sequence ID (i.e. the ID is 0). Now loads default options.
Fix: Load posts to cache on init
Fix: Error handling to return NULL if no errors present.
Fix: Only add post to sequence if it's new (never present).
Fix: Modify existing entry if the delay value changes & reset notification flag for users for this post.
Fix: hasPost() returns the location of the post in the sequence array for quicker access
Enh: Add filter for user permissions.
Enh: Filter for CPT labels 'pmpro_seqence_cpt_labels'.\
Enh: Only attempt to load posts & delays for Post/Page metabox if the sequenceID isn't 0 (invalid)