Try with make

I'm opening this PR for discussions. This PR suggests replacing `zc.buildout` by `make`.

By using `make` we gain a lot of flexibility. `zc.buildout` is good for Python projects, but is not that useful here.

And by using `make` as done in this PR we can easily solve #14 (by installing the closure_linter in the venv the `Makefile` creates).

Another advantage is that `make` only builds what needs to be built (by comparing file modification dates), making the developement process leaner and more efficient.

The `Makefile` is also really easy to use. Here's the output of `make help`:

```
Usage: make <target>

Possible targets:

- css       Build CSS
- js        Build JavaScript
- deps      Build deps.js (for script autoload with Closure)
- index     Create index.html and index-prod.html
- lint      Run the linter
- test      Run the JavaScript tests
- all       All of the above
- clean     Remove generated files
- cleanall  Remove all the build artefacts
```

See also https://github.com/geoadmin/mf-geoadmin3/blob/dev_makefile/README.md.