fixed bugs to make the flip_sign option work correctly
fixed bad behavior when working with sparse variableStep data from BigFiles
added the interval_method option for statistical summary data
Merge pull request #12 from tjparnell/interval_method

Merged fixes from Timothy Parnell to flip_sign in hybrid plot and bad behavior when working with sparse wiggle data.
fixed rt.cpan.org ticket 85984
Merge pull request #15 from wsdookadr/master

fixed rt.cpan.org ticket 85984
Fixed a problem with negative score colors
For regions where there is no coverage, leave the background transparent
Correcting an issue where filled boxes are drawn when there is no BigWig data for the segment
removed external dependency for Bio::DB::BigWig when processing statistical_summary; changed name of internal method bigwig_stats to summary_stats