Refactored the response object and the request handling loop.

- Responses can now be created empty;
- Setting and getting the content type if done using headers;
- Setting the text content later on will keep it encoded to the charset;
- Charset information included in the response content-type;
- When no explicit response is returned, the existing one on the request is used.
Now able to set headers and cookies, this resolves #7.

- Request.AddHeader sets headers on the provisional response object;
- Content for the response is always passed onto the 'text' decorator, converting to string to avoid problems with templateparser's SafeString type;
- Fixed redirection bug with uweb_info; this will require a better solution.