Updating libnqsb-tls and evaluating status

[abbysmal]

Hi everyone,

Following #6, I guess this is a good opportunity for me to take a look at this project and perhaps clean a few things.
I pushed a branch to fix the Ptime issue associated to #6 (see update-tls-and-cleanup branch, feedback welcome). (also fixing some warning related to String.lowercase, I bumped the required ocaml version to 4.03.0)

Before merging I'd like to test that it does indeed still works with latest libtls version, which seems unlikely as it seems I forgot to document properly the version of the API I used.

I don't have a decent workstation to try this now (only my work laptop running Debian, and it seems Debian and LibreSSL aren't exactly friend.), I may be able to test it properly next week, or if someone want to at least run the tests in the test directory, it would be much welcome.