Discussion about Jest for unit tests

I just added a PR #32 to have an example of working unit tests with Jest.

Maybe we could use this issue for discussion if other people are interested in making jest usable with ionic. Also maybe there are good reasons not to use Jest with ionic. I would appreciate if someone could enlighten me in this case :)

Some advantages of Jest over Karma:

- Only run tests on modified files
- Only run tests for modified files since last commit

Article about testing with Jest in angular:
https://www.xfive.co/blog/testing-angular-faster-jest/