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.
Merge pull request #51 from airbnb/xplatform-inheritance

Add es5shim & es5sham for x-browser inheritance