Extended Correios.frete parameters according to Correios Webservices Manual. Updated README.markdown example and reformatted its syntax.
Removed unnecessary import of 'Cep' and 'Frete' Model classes in correios.py
Updated README.markdown example
Tried syntax highlighting in README.markdown
Added new line at the end of README.markdown - GitHub recommendation
Merge pull request #13 from arthurmaciel/master

Made Correios.frete work again and reformatted README.markdown