Add helper files to convert matlab tours to Ipython notebooks.

Takes most of the grunt work out of translating a numerical-tours
Matlab file into an IPython notebook.  It will auto-populate the
notebook with headings, markdown, code, and exercises as appropriate.
Ideally, all one then has to do is translate the code itself.

Some code transformations are in place (see `CODE_REPLS` and `_parse_code`).