Make the new keyword optional.

[sorribas]

Hey! Great module!

This PR makes the new keyword optional, so that you can do stuff like this.

var set = require('set');
var colors = set(['red', 'blue', 'green']);

This also prevents the module from modifying global state if someone forgets the new keyword.

[gkatsev]

Thanks for the PR. And it comes with tests! Awesome.
Not sure what's up with travis.
I'll try and get this pulled in and released soon.

[sorribas]

The tests failures are related to the old npm version that cannot recognize hat dependencies.

If you add this to your .travis.yml

before_install:
  - npm install -g npm@~1.4.6

It should get fixed 😉