Add test suite + 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

[MattIPv4]

As noted in the issue, this should be fully testing the behaviour of the generate method, but also the private methods involved in the generation process.

See the Ruby gem test suite for inspiration: https://github.com/MattIPv4/do_username/blob/master/ruby/spec/do_username_spec.rb

[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

[MattIPv4]

Sorry for not getting to this sooner, not sure why I lost track of updates here a while back. LGTM, tyvm!