frames: add request_ts to wbinfo and use that as the timestamp in the top-frame. for exact replay, request_ts == timestamp
for latest replay / no timestamp / memento timegate, redirect to current time instead of time of last capture, while serving
last capture.
timeutils: add timestamp_now() function to return timestamp of current datetime
Add extra tests for this mode
Tracked via #72
canonical, no-modifier archival url (instead of mp_).
When using frames, the page redirects to a 'tf_' page, which then uses
replaceHistory() to change url back to canonical form.
memento: support for framed replay, include memento headers in top frame
bump version to 0.6.2
memento support enabled by default, togglable via 'enable_memento' config property
supporting timegate and memento apis, no timemap yet
supporting pattern 2.3 for archival and pattern 1.3 for proxy modes
also:
simplify exception hierarchy a bit more, move down to utils
make WbRequest and WbResponse extensible with mixins (eg for memento)
