rearranged imports header
accept_form_response() modified:

- expected_fields changed to tuple (iteration is faster)
- Replaced obsolete use of 'dict.has_key(key)' with 'key in dict'
- Use of str( in prints replaced with the string format() method.
- Removed redundant setting of response_status = "invalid pubkey" (was already set to this value)
- PEP8 fixes.