Expose, fix, rename Image url

Image below is for 1000 image URLs with the following code:
```
from bokeh.plotting import *
output_file("foo.html")
image_url(url=["http://bokeh.pydata.org/_static/bokeh-transparent.png"]*1000, x=range(1000),y=range(1000), angle=0 )
show()
```
The resulting file sized is 766k. This interface is clunky but I think that should be improved in another PR. 

![screen shot 2014-04-21 at 10 04 16 am](https://cloud.githubusercontent.com/assets/1078448/2755304/40c19ee0-c966-11e3-8f79-45eb4b3b6864.png)