Require 'new' operator to construct form data. (Browser parity)

[DylanPiercey]

This is a breaking change (3.x), although most places are probably already using the 'new' operator anyway.

In the browser directly calling FormData throws and this PR mimics that.
Working towards #124.

[coveralls]

Coverage increased (+0.5%) to 98.492% when pulling b8e4aac on DylanPiercey:dom-parity-require-new into 9bfb102 on form-data:master.

[jimmywarting]

Think you should remove .DS_Store and package-lock.json

Other then that, looks good

[yordis]

I believe that if we move to ES6 classes this could be fixed but don't quote me on it.

But we should follow the spec for sure.