Adds tests + formatting for bindTo()

[ashfurrow]

Mergers #70.

I was initially confused about how to use this, and the Demo came in handy! I'm definitely a fan of this approach, great work @stefanomondino!

A remaining question: should the action parameter in bindTo() functions be optional?

We should also update our readme once this is merged 😉

[ashfurrow]

@stefanomondino you can review my individual changes here: https://github.com/RxSwiftCommunity/Action/pull/72/files/747917a73ec992da38e329ec94d7cb7bebc491fc..c62ca31ffded0817b3989b0cefc6ebdd312072f1 Everything I did is up for discussion, so don't be shy!

[stefanomondino]

@ashfurrow I was thinking about cases when you want to "unbind" the Action from the button, that's why I've declared the action as optional. Felt better than passing something like Action {_ in return .empty()}.
If it was just for me, I would have declared a second property on each control with a new type, something like var controlAction:ControlAction? , where ControlAction is a NSObject subclass that takes an Action and a input transform closure (cloning ReactiveCocoa's CocoaAction), but I'm afraid it can generate too much confusion.

[ashfurrow]

Cool, @stefanomondino if you could take one more look and merge if it's looks okay, that'd be 👌