Remove global `React` lookup during runtime

This removes the global `React` lookup that was happening within the
React Intl Components' `render()` methods. A new local `react` module
has been created to provide the reference to the `React` object.

**Note:** This is a temp solution, the real fix we want is to update
the ES6 Module Transpiler to poperly deal with external, non ES6
modules.

Closes #47
Merge pull request #48 from ericf/remove-runtime-global-refs

Remove global `React` lookup during runtime