Unit Test for lib/user.js

I've implemented some support for unit test in the project. Right now, we will have to run a test redis instance to run the tests. When @at-kevinlau integrate fakeredis to the project, we will be able to point the test_db to use fakeredis instead of a redis instance. Instructions to run test is updated in the README.md file.

So far, I've only added tests for most of the main user library function. Next steps will be to integrate persona-faker in the test (to test authentication and login), and team related functions. I've also skipped the tests that have failed for now (mostly are failing because of inconsistency issues)

r? @cvan @at-kevinlau @aricha