Reorganize files, fix pep8 lint, add comments
Drop create_csv from API

It's not clear what the use of this function is. Let the caller create the file.
Add support for modifying column headers

fixes #15 on github
fixes #17 on github

By default, verbose_name attributes on fields will be used for column headers, unless use_verbose_names is set to False.

Also, the user can provide a map, field_header_map, that will map field names to custom column headers. This will take precedence over verbose names, and can be used in conjunction with use_verbose_names=False.