Add es5shim & es5sham for x-browser inheritance

The lightest-weight way to achieve cross-browser class-style
inheritance is to use `Object.create()`, polyfilled by `es5shim` and
`es5sham`.

The polyfill to `Object.create` doesn't support the second parameter,
so set the constructor manually.

This attempts to solve https://github.com/airbnb/rendr-app-template/pull/45