HTSlib as a git submodule

Hi Richard,

As discussed in #7, here is a patch to add HTSlib as a git submodule. It includes brief documentation on installation using git.

Currently, it is tied to the HEAD of HTSlib. It may be advisable to tie HTSlib to the 0.2.0 release, when it is finalised. If you agree, let me know and I'll update the PR when that version of HTSlib is released.

Thanks,
Kevin