Set Git attributes similarly to the samtools repository

Mark BAM files (if we add any) as binary and view their diffs as SAM.
Omit control files and README.md from release tarballs.
Run tests using htslib's just-built bgzip and tabix

Use makefile dependencies to ensure bgzip and tabix are up to date,
and (typically) run them via explicit paths to avoid falling back to
already-installed copies.

Add test/test.pl infrastructure to supply explicit bgzip/tabix paths
as required.  Eventually, there will be a Makefile mode to use system
bgzip and tabix along with a system htslib; such a mode will provide
empty $(BGZIP) and $(TABIX) and so "test.pl --exec bgzip= --exec tabix="
invokes executables named "bgzip" and "tabix" via $PATH.