refactor($sniffer): make $sniffer service private

[IgorMinar]

This service has been accidentaly documented in the past, it should not be considered
to be public api.

I'm also removing fallback to Modernizr since we don't need it.

Breaks any app that depends on this service and its fallback to Modernizr, please
migrate to custom "Modernizr" service:

module.value('Modernizr', function() { return Modernizr; });