Took two pull requests, one for Swift 3 and one for fixing a macro name.

Adds requested support for watchOS and tvOS (#3)

• Fixed a minor compiler warning (#2)

• Rename the INTUDefines.h file to INTUGroupedArrayDefines.h

• Add a missing header import to one of the internal header files

• Adopt the new Generics and Nullability Annotations in a backwards-compatible fashion

The initial release of INTUGroupedArray.