Camlp4 is now an optional dependency on Cstruct

This release moves the camlp4 extension to being an optional dependency, so that libraries that manipulate Cstruct values without actually specifying C structure layouts are not forced to introduce a camlp4 dependency.

• Make camlp4 an optional build-time dependency (#35).
• Remove ounit as a dependency in the opam file.
• Improve opam description file for OPAM 1.2 workflow (#36).
• Refresh Merlin IDE description (#37).