Integrate travis-ci

I added two files at the root: `.travis-ci.sh`  and  `.travis.yml`, modifed versions of @avsm's ones (c.f.
[his blog post](http://anil.recoil.org/2013/09/30/travis-and-ocaml.html)).

They run:

    omake
    omake test
    _build/benchmark/biocaml_benchmarks
    omake doc