print statements go to events log, bottle listener is quiet by default

* New pytest commandline option: `--event-testing-verbose-listener`,
  probably does what you think.
register_event fixture hooked up to the uses_event_listener mark
setup_providers adds all provider before validation
Merge pull request #510 from seandst/no-event-prints

print statements go to events log, bottle listener is quiet by default
Merge pull request #511 from seandst/setup-providers-speedup

setup_providers adds all provider before validation
Bump marker fixture scopes to session
Merge pull request #512 from seandst/fixture-marks

Bump marker fixture scopes to session
detect_observed_field bails if an element isn't visible
Merge pull request #513 from seandst/observered-field-noelement

detect_observed_field bails if an element isn't visible