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.
Shore up ipynb examples

Refer to master branch for urls, use common configurations
for power round example.
add asymptote gallery links
Merge pull request #1 from azjps/master

Some additional features and examples for ipython asy magic