Add TextStyle offsets getters to API.

The other properties getters are in the API already.
Thank you for the information @elemoine.
I updated the PR.

On Wed, Sep 03, 2014 at 07:15:40AM -0700, Éric Lemoine wrote:
> > @@ -64,13 +64,13 @@ ol.style.Text = function(opt_options) {
> >  
> >    /**
> >     * @private
> > -   * @type {number}
> > +   * @type {!number}
> 
> This is not necessary. See https://developers.google.com/closure/compiler/docs/js-for-compiler?hl=fr#nonnull.
> 
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/openlayers/ol3/pull/2677/files#r17051619

-- 
Guillaume Beraudo
Camptocamp Engineer