Async conversions
Use Object as argument type for fromFragment helper

This is to prevent compilation failures in projects that do not link the support-v4 library. The only other option I saw to fix this would have been to split up the android module into one for native fragments and one for support fragments, which I found would come with unjustified overhead. Hence I settled for the cheap way, which is to sacrifice type safety here.

Refs https://github.com/soundcloud/RxJava/issues/2
tests for zipping a finite and an infinite Observable
Adding utility functions for observables of strings useful for processing non blocking IO.
Moving StringObservable to contrib module
moving the AssertObservable class to the test side of the build.
setting the up that string test to use core test
Operation SkipUntil
Inner classes set to private
Fixed Zip issue with infinite streams.
Tests to verify fix and error behavior
Fixed case with skipUntil(empty()) == never() (matches Rx.NET)
Merge pull request #516 from abersnaze/string-observable

Adding utility functions for observables of strings useful for processing non blocking IO.
Merge pull request #531 from Netflix/docs

visually distinguish operators that use schedulers, etc.
Merge pull request #533 from akarnokd/ToAsync2

Async conversions
Merge pull request #535 from soundcloud/android-support-library-fix

Fix compilation errors due to referencing the Android support library directly
Merge pull request #545 from akarnokd/ZipInfiniteFix

Fixed Zip issue with infinite streams.
Merge pull request #539 from samuelgruetter/test-zip-pr

Zipping a finite and an infinite Observable
Merge pull request #541 from akarnokd/SkipUntil

Operation SkipUntil