Add cached_convert_shape() and cached_convert_image() methods

[hx2A]

ref discussion thread for design ideas

[hx2A]

@villares , this is complete.

You can see the documentation for the two new methods on the dev website:

http://dev.py5coding.org/reference/sketch_convert_cached_image.html
http://dev.py5coding.org/reference/sketch_convert_cached_shape.html

I also added both to the reference summary page you made because I figured you'd want that.