fixes and improvements

- fixed second prompt overlong indentation with ipython console
- using default_timer to be cross-platform
- Cookie.parse as a staticmethod instead of classmethod
- fixed minor bug with RequestSet.__call__() not clearing previous responses even with force=True
- one-digit thread numbers left padded with a space for better readability
- every python object that is pickable is now stored on disk
- can now inject in headers
- not encoding payloads by default as it depends on where the payload will be inserted
- using standard python library for urlparse.parse_qsl and urllib.urlencode instead of duplicating the code (simply needed to pass the right options to the standard functions)
- using collections.OrderedDict for parse_qs to keep the same order of HTTP parameters