Move example deps into devDependencies

As far as I can tell, `angularjs-nvd3-directives` doesn't depend on `moment` or
`angular-route` directly; rather they're only used in the examples.

Build tools such as `grunt-bower-install` will inject each dependency's `main`
properties into `index.html`, which in our case, is unnecessarily including
`moment`.
Merge pull request #147 from tlvince/moment-dev-dep

Move example deps into devDependencies