Add identify method support

[ookami-kb]

Requirements

• [x] I have added test coverage for new or changed functionality
• [x] I have followed the repository's pull request submission guidelines
• [x] I have validated my changes against all supported platform versions

Related issues

Fixes #24

Describe the solution you've provided

Added identify method. I've also updated the way anonymous user are treated: if null is passed as userKey, a new UUID will be generated and passed (together with setting isAnonymous parameter to true).

How to test?

Log in/Log out button was added to the example app that switches users context.