allow from range_regex import regex_for_range as documented

First of all, thanks for sharing this excellent module; very useful for generating regexes to be used in apache 2.2 config from a collection of ip address ranges.

It looks to me like range_to_pattern was intended for internal use only, so I just replaced it with range_for_regex to allow `from range_regex import regex_for_range` as specified in the project's readme. Please forgive if I've got this all wrong :anguished: 

Thanks again!