Reenable idxstats and convert to htslib

Reenable idxstats code by using new functions added to htslib.
Convert API for index and idxstats to htslib.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Update bam_index.o header dependencies
Merge idxstats htslibification (PR #191)
Call hts_set_fai_filename on the output fd too so that -T has a chance
of working. This is one half of the fix necessary for
https://github.com/samtools/samtools/issues/144
Merge QS-vs-PL branch into develop