Intercept redirect only if it's not XHR (Ajax)

Intercept redirect can't really help debugging at all
if requests are XHR (Ajax), because browsers just don't render
their response by default.
support logbook: implement LogbookPanel

details:
- create new module flask_debugtoolbar.panels.logbook
- it contains LogbookPanel, ThreadTrackingHandler
Merge branch 'ajaxfix' of github.com:crosspop/flask-debugtoolbar