Updated return types on common methods

Updated return types for:

`centerInView:`
`centerInContainerOnAxis:`
`pinAttribute: toSameAttributeOfView`
`pinEdge:toEdge:ofView:`
`pinEdge:toEdge:ofView:inset`
`constrainToSize:`

To return either the NSLayoutConstraint or a NSArray containing the NSLayoutConstraint's created in that method

Updated the Podspec to use tag 0.0.3

Updated the Readme