Add examples for -scan… and -mapPrevious…
Rename -mapPrevious… to -combinePrevious…reduce:
Rename -scanWithStart:combine: parameter to `reduce`
Rename -aggregateWithStart:combine: parameter to `reduce`
Rename -fold…combine: parameter to `reduce`
Explain how -mapPrevious… is different
Merge pull request #667 from ReactiveCocoa/clarify-map-previous

Rename and clarify -mapPrevious…, rename all combine: parameters to reduce: