Switch to an oasis-based build-system

This a pretty big commit

- 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`.
- 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),
- and the documentation and its generation were updated.