Functional tests

[kangz12345]

As discussed in #35, we are going to add functional tests along with unit tests.

Functional tests verify the public API specification, hence they can have several dependencies to other classes, modules, or even external libraries.
We can implement such tests using unittest module, just like the unit tests we have implemented so far.

Although the unit tests are mandatory, the functional tests are not. However, we recommend to implement the functional tests for complex or important features.