Splitting externals

The  `buildout` _ol3-serverconfig_ is run on all server, even when deploying. It generate two build environment dependent files  `serverconfig.js`and `layersconfig.<lang>.js`

The parts `ol3-update`is run only at will.

If anybody knows how to correctly escape the content of `layersconfig.<lang>.js` into the loader's `<scri' + 'ipt>`tag, I would be happy

So if you want to merge:
1. Test this branch against geoadmin/ol3/tree/dev_split_externals
2: If OK, merge https://github.com/geoadmin/ol3/pull/63
3. Merge this PR
4. Make a branch a run `./buildout/bin/buildout -c buildout install ol3-update3` and PR the result
5. Merge this second PR