Switch to an Oasis-based build-system

This a pretty big commit, the previous commit introduced
a link error because it broke 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 `_oasis.in`.
- 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 the documentation and its generation were updated.