Clean up datasource API.

Add proper docstrings for published BaseDatasource methods using
Google's docstring standards
(http://google-styleguide.googlecode.com/svn/trunk/pyguide.html?showone=Comments#Comments)
.

Remove the standardized_filename and filename_from_url methods.
These are replcated on openelex.base.fetch.BaseFetcher.  The more
relevant implementation for the datasource is
BaseDatasource._standardized_filename.