Add ol.proj.Simple

This PR adds a new projection class, `ol.proj.Simple`, for cases where a real geographic projection is either not needed or not available. Generally this is for cases where ol3 is used to display non-georeferenced data, for example a [Zoomify image](http://openlayers.org/dev/examples/zoomify.html), [game maps](http://www.ign.com/maps/the-elder-scrolls-5-skyrim/whiterun), or [fractals](http://jsfiddle.net/D_Guidi/CjCea/). The idea is that such a projection can be used as a "stub" projection for such maps and sources.

@ahocevar, I'd particularly appreciate your feedback on this.