WebAgg: reuse already running Tornado ioloop + optionally use logging instead of print()

If a Tornado ioloop is already running before WebAggApplication.start() is called,
it should be re-used instead of trying to start it again. This allows one to
use WebAgg in his/her external Tornado application.
Additionally, I added webagg.use_logging option to turn on logging instead of
print(), which helps to integrate WebAgg in a spherical production application
in vacuum.