Write library version in build files

This PR adds the most recent Git tag (`git describe --tags`) to the build files.

The `ol.js` now looks like this:

    // OpenLayers 3. See http://ol3.js.org/
    // Version: v3.0.0-gamma.3
    (function(){var ...

And `ol-debug.js`:

    // OpenLayers 3. See http://ol3.js.org/
    // Version: v3.0.0-gamma.3
    var CLOSURE_NO_DEPS = true;
    ...

If there are further commits after the tag, you will also see the last commit. For example, after I made this commit, the version changed to `v3.0.0-gamma.3-1-g076f53c`.

Note that you'll have to run `git fetch upstream --tags` to get all remote tags.

closes #1792
Minor, but I think we should omit the version line altogether if it cannot be derived.