Add distinctConsecutive{,By} to process1

This one adds `distinctConsecutive` and `distinctConsecutiveBy(f)` to `process1` which remove consecutive duplicates from the input such that the first element of those duplicates is emitted.

RxJava has the same operation and they call it [distinctUntilChanged](http://netflix.github.io/RxJava/javadoc/rx/Observable.html#distinctUntilChanged%28%29). Suggestions for better names are welcome!