corrected bug in features positions after delete

fixed a bug:
_coord_adjust_deletion: test for features overlapping only one end of
the deletion site had incorrect use of Bio::RangeI::contains, which led
to incorrect adjustment of positions ifor features that start at the
start of the deletion site and extend beyond the deletion's end point.
Added a new test case for this situation.
throw exception on negative positions

changed _coord_adjust to check for negative positions, which now lead to
an exception. Previous behaviour was to allow such coordinates but the
resulting adjustments were incorrect because they were treated as
positive numbers
Merge branch 'master' of git://github.com/fschwach/bioperl-live into topic/pull_request33

Conflicts fixed (and perltidied):
	Bio/SeqUtils.pm
	t/SeqTools/SeqUtils.t