Allow single-base intervals in GFF where the start position equals the e...

The GFF specification used by this module (http://genome.ucsc.edu/FAQ/FAQformat.html#format3) says that the end position of a feature is to be specified as inclusive. To me this indicates that a single-base interval should be specified with the same start and end positions, and that such intervals are not interpreted as null by the GFF spec. This fix merely allows for such GFF files to be read by track. Thanks very much!