Some additional features and examples for ipython asy magic

* %%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 a couple of examples, such as scraping and compiling asymptote code from the asymptote gallery html