Allow the user to define a custom bar button for done and cancel

[MattCBowman]

The bar button's target and action properties are weakly protected because they are overridden in the setters, but they could be changed after the fact if the user retains a handle to them.

[rhfung]

Would you like to explain how your solution is different and better than #82? Perhaps identifying a use case would be beneficial.

[MattCBowman]

Hey Richard - the goal of this item was to allow additional control over the default cancel and done UIBarButtons. In our case, we wanted to use FontAwesome icons for the buttons, so we needed to set the font on the buttons, not just alter the text (as seen in #82).
After discussing it, our team decided that it would be a pain to try to make methods to support all of the customizations a user might want for the bar buttons, so we went for the more expedient route, allowing users to pass in their own UIBarButton. Here's an image of the result:

[skywinder]

Please, remove Podspec file from pull request.

[ma2kubo]

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[skywinder]

Before merge it needed to add example of new methods usage to README.md file.

Thank you.

[ma2kubo]

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Subject: Re: [ActionSheetPicker] Allow the user to define a custom bar button for done and cancel (#87)

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[skywinder]

Merged in skywinder/ActionSheetPicker-3.0@6e873f7

[ma2kubo]

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Subject: Re: [ActionSheetPicker] Allow the user to define a custom bar button for done and cancel (#87)

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