Change the order of the Interactor#execute() method parameters

[DmitriyZaitsev]

Now all of the execute methods have requestModel the last parameter:

no param	w/ param
execute(onNext)	execute(onNext, requestModel)
execute(onNext, onError)	execute(onNext, onError, requestModel)
execute(onNext, onError, onComplete)	execute(onNext, onError, onComplete, requestModel)

This is not friendly with Kotlin where if the last parameter of a function accepts a function, a lambda expression that is passed as the corresponding argument can be placed outside the parentheses.
So, now in Kotlin we need to write:

interactor.execute({ item -> doSomething(item) }, param)

instead of:

interactor.execute(param) {
    item -> doSomething(item)
}

At the same time, the current order causes issues with correct type inference.
I think, it's reasonable to make responseModel the first parameter.