Use `append` to append characters to a String

`+=` no longer appends characters, as of Xcode 6 beta 6.
Change ArrayZipper to satisfy `ArrayLiteralConvertible` conformance requirements

* Change return type to `Self` in `convertFromArrayLiteral` implementation.
Assuages Xcode 6 beta 6's complaints about the implementation in a non-final class.

* Mark the initializer `required`, as required by the `convertFromArrayLiteral` implementation.
Fix `Maybe` `==` implementation for Xcode 6 beta 6
Merge pull request #100 from brendonjustin/beta6

Update for Beta 6