Refactor BW::UIBarButtonItem module to a class

[ryansobol]

So, I've come to a realization. I think that changing BW::UIBarButtonItem to a subclass (rather than a factory module) is a better architecture pattern for BubbleWrap's future.

My original design for BW::UIBarButtonItem was a factory module because:

1. I was concerned that overloading +init on UIBarButtonItem would be dangerous.
2. Someday, I wanted to explore the option of polluting UIBarButtonItem itself.
3. I disliked the thought of typing BubbleWrap:: all the time.

However, I've since changed my mind while implementing BW::UIAlertView:

1. lrz reminded me that overloading .new on Cocoa Touch classes is quite safe.
2. I've given up on my dream to pollute Cocoa Touch classes because of the potential compatibility issues it creates with other 3rd party libraries.
3. I've actually grown found of seeing BW:: in my code. :)

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This patch has no behavior changes and maintains backward-compatibility. The only changes are:

• The constructor methods now return instances of BW::UIBarButtonItem which is a subclass of UIBarButtonItem.
• The .new constructor method replaces .build. Calling .build forwards to .new and outputs a deprecation warning to NSLog.

[clayallsopp]

Awesome! Makes a lot of sense, brings it in line to BW::UIAlertView.