make npm installable / requireable

[rt2zz]

I think this would be a lot easier to experiment with if we make it npm installable & requireable

The basic changes required to make this work can be seen here: https://github.com/rt2zz/navigation-rfc-module/commit/089aa9bb8ec3745d6f06dbec6d6d0c899d90eb88

It could then be installable as follows: npm install ericvicenti/navigation-rfc and require: var Navigation = require('navigation-rfc')

If you are open to this I will submit a PR

[rt2zz]

note: we can also go one step further and remove the react-native injection and require people to require navigation components straight out of navigation-rfc