this PR - best viewed at the individual commits, does the following:

• exports X509.Encoding.distinguished_name_of_cs -- similar to Add cs_of_distinguished_name #87 which provided distinguished_name_to_cs
• exports X509.Encoding.{public_key_of_cstruct,public_key_to_cstruct}, as requested by @dinosaure
• support of cstruct 4.0.0, which split up the sexp de&encoders
• removes result dependency (now requires >= 4.03.0)
• upgrades opam file to version 2.0
• moves the build system to dune
• require ocaml 4.04.2, since 4.03.0 and ppx and dune does not seem to be friends (see https://travis-ci.org/mirleft/ocaml-x509/jobs/514649801 in case you're interested)