Fix strand-based Gap attribute processing for segments Glyph 

While using GBrowse 2.55 to visualization the comparison of gene models represented in the GFF3 canonical gene format with alignments of transcript sequences from gene models of an earlier genome assembly, and represented in GFF3 with the Gap attribute, I noticed the exon ordering of some of the alignments was reversed compared to the corresponding gene models:

![before](https://cloud.githubusercontent.com/assets/1800812/5038733/e4c691cc-6b59-11e4-8901-04e0c233be2e.png)

Note that the top track is the canonical GFF3 gene format, while the GFF for the bottom track was created 
 

![after](https://cloud.githubusercontent.com/assets/1800812/5038741/ea4ac4ba-6b59-11e4-8043-710441cfaeb7.png)

From my understanding of reading the GFF3 spec & SAM specification, it looks like CIGAR operations are always defined with respect to the reference + strand, so the logic shouldn't differ for + and - strands. I could be misinterpreting the specification and/or code, though, so please check the code in this pull request carefully!