Fix compiler warning in stats

Change fileno((FILE *)stdin) to STDIN_FILENO.
Add include <unistd.h> for isatty(int) we lost it when I stopped
including sam.h

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Enable tests for samtools stats

Fix a typo in stats usage message.
Create and enable tests so that samtools stats is tested when make check is run.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Merge pull request #179 from mp15/stats

Stats unistd fix and tests enable