Scale and anchor ImageURL and add an example

Now you can specify `w` and `h` for width and height. If not provided, image size will be used (the original behavior). Also you can specify `anchor` to override default top-left positioning. See the example for details. `anchor` possibly should be called `position`, at least this is what `distance_vector()` uses, and merged with `distance_vector()`, so that units are managed properly, but the code there didn't encourage me to do this right now.

This is related to https://github.com/ContinuumIO/bokeh/issues/566.