added calls onEnterForeground & onExitForeground

per comscore support they recommended using onEnterForeground &
onExitForeground in our apps instead of onUxActive & onUxInactive
Update index.md

added entries for onEnterForeground, onExitForeground & additional information about the indended usage of onUxActive
Merge pull request #1 from n3wc/develop

added calls onEnterForeground & onExitForeground