Provide access to all PySAL map classification methods

This PR uses the `imp` library to dynamically load the requested PySAL map classification scheme, which no longer restricts users to the three hard-coded classification methods previously provided. It'll default to 'Quantiles' as before if the requested method isn't found.

Caveat:

`imp` is pending deprecation (though included) as of Python 3.5, and the use of `importlib` is recommended instead, but Python 2.6 doesn't include `importlib`, though it's available to be installed from PyPI, and *could* be provided as a conditional requirement based on the python version.

More generally: should this functionality be moved to `geopandas.tools` (if only to discourage the proliferation of choropleth maps?)