add test suite and test example for JS

[luizcieslak]

Using jest as the test engine.

Also add a test for the function, asserting it's maxSize argument.

Closes #17

[luizcieslak]

sorry @MattIPv4, did not pay attention to that. I add all the tests the ruby package does expect for the srand one.

Besides that, I did the following:

• Add constants.js to make mock possible. See 1, 2 for more info

• export private functions in module in order to test them too

• combineUsername throwing error related to maxSize

• fix merge conflicts with master

• fix eslint errors after its implementation