Fixes #12 by removing specific constraint code

[m-revetria]

Fixes issue #12.

The crash is caused because ActionCell is changing the priority from some constraints from Required to Optional and viceversa when they are already installed. This can happen when the setup method is called twice, and the second time passing a null image. These constraints control how the cell's title label is positioned taking into account if the cell has or not an image.

I just removed these constraint from the base class ActionCell instead of trying to change the constraints properties in some way. I opted for removing them because they add specific dependent code on how the cell's UI is designed and should be out of the scope of this class. This way we are avoiding coupling XLActionController base code with specific actions' UI code.

To see how this can be implemented, developers can take a look to TwitterCell

Note: this is a breaking change. If this PR is merged then, apps that are using these constraint it will crash with the exception "this class is not key value coding-compliant for the key..."

2016-09-05 14:44:22.837 Example[6518:267146] *** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<Example.YoutubeCell 0x7fc469e62320> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key actionTitleLabelConstraintToContainer.'

[mtnbarreto]

imageViewWidthConstraint.constant = image != nil ? 36 : 0