Build for watchOS and macOS

[stefanhp]

Hi,

I'm Stefan from Myotest.

We are using your code within a framework of our own, that has build targets for iOS, watchOS and macOS. We therefore extended your project to also provide build targets for these platforms, including tests.

Here is a summary of the changes:

• Fixed a compile issue on watchOS (which triggered those changes)
• Add build targets for macOS and watchOS
• Add test target for macOS
• Add a Travis configuration so that test are automatically run with a push/PR. In order to avoid to be killed by Travis for one very long performance test, created dedicated schemes to be used by Travis that skip that test.

Let me know if you would like other changes, maybe add a tvOS target as well.

Stefan