Switch to an Oasis-based build-system

This a pretty big commit, the previous commit introduced
a link error because it broke yet another assumption of `ocp-build`:

- the build process was replaced by a `Makefile` with new or updated
scripts in `tools/`, among which `tools/please.ml` which generates the `_oasis`
file from `tools/_oasis.in`,
- we're now using `ocamlbuild` which is much slower than `ocp-build`,
- this makes the issue [#17](https://github.com/hammerlab/ketrew/issues/17)
obsolete,
- the travis build was improved and a lot of problems had to be
fixed (especially with the OSX build machines where unrelated `brew` issues
emerged, see [opam#1853](https://github.com/ocaml/opam/issues/1853), and
[ocaml-travisci-skeleton#15](https://github.com/ocaml/ocaml-travisci-skeleton/issues/15)),
- the `.merlin` file is no longer auto-generated (for now),
- everything works now with OCaml 4.02.0 (the issue was *only* `ocp-build`),
- and the documentation and its generation were updated.