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
simplified error handling as this is already done on the outside
Merge pull request #380 from jmhofer/distinct-with-comparator

Implemented `distinct` and `distinctUntilChanged` variants using a comparator