Make installation smother for systems that have system installs of samtools

Hi,

Thanks for an awesome project!

This is a fairly trivial pull request, simply making it easy to use the repository versions of samtools and libbam on linux distros that supply them (which most do). I felt like this was necessary as the instructions in install.md took some poking to get working on my system (It didn't automaticaly find sam.h, even with the unzipped download path exported).
This PR also adds the klib repository as a submodule, as this is not shipped with repository versions. I've updated the install.md instructions to reflect this.

I've done my best to ensure that the `wget` way of getting samtools still works after these changes; it certainly works on my system (Debian Sid).

Cheers,
Kevin