Update TribbleIndexedFeatureReader.java, TabixReader.java, and
TabixFeatureReader.java to add new constructor with given index file.
Update AbstractFeatureReader.java to pass
index file forward to specific readers. Pass index file in AbstractFeatureReader
to underlying readers.
Merge pull request #69 from samtools/pull_62

Update TribbleIndexedFeatureReader.java, TabixReader.java, and