Support tile sources without configured projection

This change adds a lot of flexibility to working with tile
layers: Sources where the server projection or tile grid do not
matter can now be constructed without specifying a projection or
tile grid.

The tileUrlFunction/imageUrlFunction now also creates updated
URLs when the params of the layer change, so things like
mergeNewParams in ol2 will be possible.

A nice side effect of this whole change is that there is no more
duplicated code between tiled and single image WMS layers.

While I was at it, I also fixed a WMS 1.1.1 axis order issue
and incorrect STYLES params (STYLES=& instead of STYLES&).

src/ol/source/source.js

@@ -38,7 +38,8 @@ ol.source.Source = function(sourceOptions) {
    * @type {ol.Extent}
    */
   this.extent_ = goog.isDef(sourceOptions.extent) ?
-      sourceOptions.extent : sourceOptions.projection.getExtent();
+      sourceOptions.extent : goog.isDef(sourceOptions.projection) ?
+          sourceOptions.projection.getExtent() : null;
 
   /**
    * @private