Add unit tests to all API calls

[tnachen]

There is no tests in the scheduler atm. We need to at least have simple coverage to test correctness

[swhsiang]

What kind of tests we are going to have?

• For API endpoint test: https://github.com/matryer/silk I have used it before. The experience is good. However, it is hard to integrate with Travis (travis couldn't detect the failure of the API test).
• BDD style testing framework: https://github.com/franela/goblin A Mocha like BDD testing framework for golang.
• testify. A extension package for standard testing package. https://github.com/stretchr/testify From my point of view, it is useful in our case without increasing too much complexity of this project.