Refactor/transform strategy

[vadymmarkov]

There is a list of changes that I'm planning to include in the next major release.

1. Add Travis.
2. Use SwiftLint to enforce Swift style and conventions.
3. Refactor project structure and code.
4. Remove public and open access modifiers to make public API cleaner.
5. Breaking API change: Remove transform from Config, as @glaurent suggested. From now transformer should be specified in order to conform to Estimator protocol:

protocol Estimator {
  var transformer: Transformer { get }
  // ...
}

5. Update README.

[vadymmarkov]

@glaurent I've got a chance to test demo with YIN algorithm on my acoustic guitar and it works way better than HPS 🎉

[glaurent]

@vadymmarkov yay 👍