Manage dependencies using submodules

[cyenyxe]

I have found out that all our dependencies are now maintained in GitHub. To be as up-to-date as possible, it could be a good idea to manage them with submodules. If we point to the tag with their latest release we can also guarantee stability.
From the point of view of the applications that depend on hpg-libs it shouldn't be a problem because the git submodules command has a --recursive option that handles nested submodules.

The links to the projects are:
https://github.com/samtools/samtools, https://github.com/samtools/htslib (tagged releases)
https://github.com/akheron/jansson (tagged releases)
https://github.com/mackyle/sqlite/ (unofficial, tagged releases)
https://github.com/argtable
https://github.com/hyperrealm/libconfig

Thoughts? :)