Added Swift Package Manager support

[KaneCheshire]

Fixes #40

Before merging, need to make sure this works with Xcode 11 / Xcode 12, and also that you can still use Cocoapods and Carthage without any problems.

[KaneCheshire]

Xcode 12 SPM:

Note: to use the biometrics stuff directly, you need to import Biometrics at the top. If you're just using the helper functions in BiometricsContext, you can just import TABTestKit because BiometricsContext.swift imports Biometrics.

[KaneCheshire]

Xcode 11 SPM:

Also noticed you can create an instance of Biometrics with Biometrics.init. There's no harm since it doesn't do anything, but we can probably hide the init to make it a bit more obvious you don't create an instance and just use the class methods.

Should also update the README with info specifically for SPM, like needing to import Biometrics

[KaneCheshire]

Xcode 11 Cocoapods:

Biometrics import not needed

[KaneCheshire]

Xcode 12 Cocoapods:

[zacoid55]

@KaneCheshire well this is damn exciting! Code looks good but it looks like Travis is having issues with the tests, specifically around Biometrics

[KaneCheshire]

@zacoid55 yeah I think it's the Carthage checks that are failing. Although the last build on develop also failed when the last PR was merged :/

[KaneCheshire]

Ah it's trying to build the package on Xcode 10.* which will never work because SPM is 11+. Will have to look into how to only run for one tomorrow

[KaneCheshire]

Now works with Carthage too 👌

[KaneCheshire]

Hmm one of the UI tests is failing on Xcode 11

[KaneCheshire]

Passed this time, looks like we might have a flake. Merging now