Include htslib in our build directory

Eventually perhaps we'll be able to install an htslib-dev package
during before_install instead.
Include htslib rules and add header dependencies

Use the rules and variables from htslib.mk to write out exact
dependencies on htslib headers and (for executables) library.
With this, samtools builds with "make" from here are correct
even when htslib source code is being modified.

(A little more infrastructure is needed to select between static
and shared libhts.)
Added BGZF indexing and reindexing capability to bgzip. Fixed -b seeking.
Added a test suite for samtools, currently faidx and bgzip are tested.
Added a test suite for samtools, currently faidx and bgzip are tested.
tview and bgzip: Check that the files are bgzipped, not gzipped.
Fix buglet: initialise the kstring_t

Also parse "" rather than NULL in the unlikely event of an empty .fai
file, and add a bam_import.o prerequisite that was omitted.