bugfix and new behaviour with negative coordinates

As discussed on the bioperl mailing list, I have modified Bio::SeqUtils::_coord_adjust so that it will throw an exception if negative coordinates are seen. Had to slightly change the existing code to allow checking for negative coords.
Also fixed a bug which cause incorrect postion adjustments from Bio::SeqUtils::delete action if a feature started at the same position as the deletion and ended beyond the deletion's end.
Aded a few tests for these cases.