Do not use experimental JavaScript

[balloob]

Using experimental JavaScript is not the right way to go. It will change until it is finalized and thus will cause us to have to update our code in the future. It also forces us to use babel-eslint which has blocked us in the past from upgrading to a newer Babel version.

I suggest we limit Babel to only the presets es2015 and react.

• Add package with polyfill for Object.assign and use that.
• Rewrite decorators to function calls
• Other stage 1/2 stuff that sneaked in

[gthomas-appfolio]

Added related #85