Use opam pin for development

There is a (new?) feature of OPAM, which makes opam-bleeding unnecessary. See the README file for details.
The short version is that we can add an `opam` file to this repository itself and simply use `opam install packet` and `opam remove packet`. This will pull in all dependencies, etc. This is much more robust than using the Makefile and asking the user to manually install dependencies.

@jnfoster @seliopou