Add task for publishing to npm

This task publishes an existing tag to the npm registry.  To publish a new release, create a commit that updates the version number in package.json (e.g. to "3.1.0").  Then create a tag, push to GitHub, and run the publish task.  Assuming "openlayers" is the remote for the canonical repo, this would look like the following:

    git tag -a v3.1.0 -m "3.1.0"
    git push --tags openlayers
    ./tasks/publish.sh 3.1.0

The task creates a build for each of the `PROFILES` in `publish.sh` (these correspond to `.json` files in the `config` directory).  Builds are generated in the `dist` directory.  Our `package.json` specifies `dist/ol.js` as the "main" build.  So when people use a module loader to `require('openlayers')`, they get the full build.  It is also possible to load a debug build (e.g. `require('openlayers/dist/ol-debug')`), and we can publish additional builds by adding `config` files and updating `PROFILES` in `publish.sh`.

The `.npmignore` file determines what is *not* included in the package (note that `node_modules` are always ignored).  So if additional items are added to `.gitignore` that should not be included in the npm package, they need to go in `.npmignore` as well (ideally, we don't need to generate anything else outside of the `build` directory that doesn't belong in the package).