Demo code: Use tokenize module to grab the module docstring.

The demo code helper functions strip the module docstring from the source code so that the docstring and the code can be displayed separately. Previously, the docstring was detected by iterating through the source and looking for triple quotes (once each for single- and doublel-quotes). This was buggy (e.g. it can pick up a class docstring instead of the module docstring; see [Chaco Issue #43](https://github.com/enthought/chaco/issues/43)). 

Instead, use the `tokenize` module to find the module docstring.