Add flatMap alias to mapMany

This is to match with what Java 8 is adding to Stream so we support the duality of Iterable/Collection to Observable.

http://download.java.net/lambda/b86/docs/api/java/util/stream/Stream.html#flatMap(java.util.function.Function)
Merge pull request #247 from benjchristensen/flatMap-alias-to-mapMany

Add flatMap alias to mapMany