Additions to ipython asymptote extension

- %%asymptote reduced to shorter %%asy
- subprocess to pipe asymptote error messages/stdout,
  raise RuntimeError for asymptote stack traces
- by default write images as png format, least
  asymptote configurations required. Add argument
  for changing the format
- by default write asy code and image to temporary
  files and cleanup afterwards, to avoid clutter. Add
  argument for saving code and images
- use asy -noView argument
Add IPython notebook asymptote magic examples

Add a couple of notebooks and asy files, and update some documentation.