Release script: add support for ocaml-github
Add 'opam source'

Want to patch an existing package ? `opam source --dev --pin <package>` and start hacking.
Don't wipe non-empty directories that seem to remain after uninstall

rather print a warning. By 'seem to remain' I mean eg. 'lib/<pkgname>' when <pkgname> was just removed
Remove the unused 'build-dep' package flag

A better way to handle build-packages is being discussed
Merge pull request #1500 from OCamlPro/cleanup-and-source

Add `opam source`; fix uninstall ; some cleanup