Handle multiple URLs

Needed for theodi/shared#140

We can now support multiple URLs, added by clicking a + button like so:

![download](https://f.cloud.github.com/assets/109774/2293550/8d61e82e-a06f-11e3-9a8e-b98f6a2ba559.png)

It then works in a similar way to datapackages, by creating a Package and forwarding the user to a results page:

![download 1](https://f.cloud.github.com/assets/109774/2293563/df469040-a06f-11e3-829b-530e56ec5250.png)