Add first test

[rubensseva]

SUMMARY

Make the code more testable, and add a simple test

CHANGES

• Improve file structure to more closely resemble the Go standard
• Decouple gRPC related code from the rest
• Use interface for redis client instead of using the redis client directly
• Add a simple integration test, using a redis client mock

HOW TO VERIFY

• Check that you agree with the file structure
• Run the new test, see that it works
• Run the examples from the README, ensure that it works