Add unhandledRejectionHandler

[simonepri]

Unhandled rejections shouldn't be handled in the same callback of uncaught exceptions for 2 main reasons:

1. It's undocumented!!!
2. Users cannot easly distinguish them

This PR solves the problem adding another dedicated handler function

[coveralls]

Coverage increased (+0.1%) to 97.436% when pulling 1053e16 on simonepri:patch-2 into c9605dd on Tapppi:master.

[coveralls]

Coverage increased (+0.1%) to 97.436% when pulling 1053e16 on simonepri:patch-2 into c9605dd on Tapppi:master.

[simonepri]

@Tapppi please enable Issues in the settings of this repo.
Are some ideas that I want to propose, but I'm unable to open an Issue. 👎

[Tapppi]

@simonepri I have enabled issues, feel free to propose changes.

As for this PR, I like the idea, but can you give your thought process for removing unhandled rejections from the error handler? I am not calling it a bad change, just want to verify the justification as it makes this a major version bump ☺️

[Tapppi]

Actually, you already gave your justification. I will merge this and release it as a new major version!

[simonepri]

@Tapppi Thank you.

[Tapppi]

@simonepri v2.0.0 has been released to npm

[simonepri]

@Tapppi the build is failing.
I don't think adding the package-lock.json file is a good idea, although it is recommended.

[Tapppi]

@simonepri I updated the style of the repository, which caused node v4 to fail tests because of a missing 'use strict';. Fixed it already.

[simonepri]

Good, currently I'm improving the documentation a bit.
I will open a PR when I'm ready 👍