added test with multiple subscribers
added missing mock
added another test against multiple staggered subscribers
wrapped subscription so that interval works for multiple subscribers and added a test for staggered subscriptions with publish/connect, too
initial introduction of an equality comparator into distinct implementation, still needs test
added test against comparator usage
added both new variants to observable
added comparator versions of distinct operation, too
added tests against multiple subscriptions, just to be on the safe side
implemented mapWithIndex
simplified error handling as this is already done on the outside
Merge pull request #379 from jmhofer/interval-multiple-subscribers

Make `interval` work with multiple subscribers
Merge pull request #380 from jmhofer/distinct-with-comparator

Implemented `distinct` and `distinctUntilChanged` variants using a comparator
Merge pull request #381 from jmhofer/mapWithIndex

Implemented `mapWithIndex`