Add new contains() method

[sflanders]

Awesome library. Coming from the .net world i pretty dependent on linq. Please add method to do a contains() on a predicate. Obviously you can use firstOrDefault() and check for null but would be great to have a real function.

boolean contains(Selector<T, TResult> selector) {
   // see if the element exists
}

[brunovinicius]

Thanks for the feedback. We've been on out of office for Christmas and new years and just been back. We'd definitely take a look at it and release it in the next version =)