1.1.0: promote Single + SingleSubscriber to Beta

[akarnokd]

Based on votes, Single, SingleSubscriber and Observable.toSingle can now
be promoted to Beta state.

[charliek]

I've been a really big fan of Single in our codebase. It helps document the API, reason about our code base, and helps us eliminate cases where people don't properly plan for multiple items when terminating streams.

To date we've not hit any corner cases that I am aware of, but we've not been using it too heavily yet.

+1

[beckje01]

I love Single it makes the API very understandable.

[dsvoronin]

will be so natural in Retrofit 2.0
but also starting to use it in our codebase as well

[passsy]

I'm using it since the first public @Experimental release and it makes Interfaces more understandable. My current Observable/Single ratio is 30/70

I hope all operators known from Observable will be promoted to public for Single such as defer and lift. Doing the following workaround seems unnecessary:

public Single doSomething() {
    return Single.just(..)
        .map(..)
        .toObservable() // to Observable because defer is private for Single
        .defer(...)
        .toSingle(); // back to Single because API says so
}

[artem-zinnatullin]

@passsy I am planning to use Single in production for the same reason of clearer contract of the API (though I see some cons in this too), so I'll try to bring most useful operators (defer in the list) to the Single during this week, and I hope Rx team will merge and release them as part of 1.0.15.