Implemented the 'Contains' operator

[zsxwing]

Hi, I implemented the Contains operator #31. I saw the discussion about the distinct operator #395, so I did not implement the comparator override. Please take a look. Thanks!

[cloudbees-pull-request-builder]

RxJava-pull-requests #315 SUCCESS
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[cloudbees-pull-request-builder]

RxJava-pull-requests #316 SUCCESS
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[samuelgruetter]

@cloudbees-pull-request-builder glad to see you work again ;-)

@zsxwing Nice. I also think that we don't need an overload with a comparator, because one can simply use exists instead:

o.exists(new Func1<T, Boolean>() {
    public Boolean call(T a) {
        // check if a is "equal" to the desired element using custom comparison strategy
    }
});