Unprecise transormation used in example

In the image custom projection example [1], the projection used is coming from epsg.io [2], but this definition is currently wrong. The `towgs84` parameter lacks precision. Please compare with the old definition. Now, this certainly is an epsg.io problem, but I think you should be aware of that.

[1] https://github.com/openlayers/ol3/blob/master/examples/wms-image-custom-proj.html#L49
[2] http://epsg.io/21781.js
[3] http://cdnjs.cloudflare.com/ajax/libs/proj4js/1.1.0/defs/EPSG21781.js