Update OpenLayers
Implement progress and error handling
Demo use of error and progress handler

This appears to be broken - progress indicator is only shown on initial
load, although the progress callback function is properly called.
Use XHR to load images if possible

If the btoa function is available, we use XHR to load the image, otherwise
we fall back to assigning the src to the image element. The benefit of
using XHR is that we can provide details on the error that occurred.
Merge pull request #705 from ahocevar/progress-and-error

New error and progress callbacks for the olMap component