Use object-assign module instead of es6-shim

This corrects an issue running firmata inside of a Chrome App, where es6-shim
conflicted with Chrome's CSP.

Specifically, this line was the culprit:
https://github.com/paulmillr/es6-shim/blob/master/es6-shim.js#L72

If in the future more ES6 methods are used, another solution might be found (or
a patch submitted to es6-shims).
Merge pull request #76 from hybridgroup/master

Use object-assign module as replacement for es6-shim