Add "headers" property on browser, containing the response headers.
Cleanup (pep8).
Implement file uploads.

lxml.html.formfill does not support file uploading at all.
Therefore file uploads are implemented using the mechanize MimeWriter.
This requires a special workaround using mechanize since lxml cannot store
the file in the document for submission.
Implement AT file upload widget, because the label does not work.
Cleanup docs (rst errors).
Merge branch 'jone-file-uploads'