extract fasta gives more infos (temporary in debug mode).
ToDo -- EmptyRegion
Transcript now uses RegionList for exon list (as intended)
method exons_on_region(utr) removed
upstream/downstream operations added to Region & RegionList; include_position? for RegionList; Splicing code refactored a bit (using upstream and include_position? terminology); collect_cages now live in Region class as #load_cages. Region#load_sequence now caches result(while it may not be an optimal solution: external caching is better