Utility Class for Accessing Unsafe

While working on code for v0.20 I have come across reasons for using sun.misc.Unsafe (such as #1372).

This provides a common way for getting access to `sun.misc.Unsafe` as well as utility methods for commonly needed APIs (that exist in Java 8, but not Java 6).

I need someone to help me confirm whether this works on Android, and if not how we can make this be aware of what platform it is running on, as per https://github.com/Netflix/RxJava/issues/1372#issuecomment-46717293