Create admin UI

[rmccue]

Right now, the Importer is missing a key component: the web interface. It's only runnable through WP-CLI right now. This sucks :(

I'd like to go further than just replicating the existing UI in the older version of the importer. I think we can make some real strides forward in providing a better UI, esp. with the resumable parsing we have. We should be able to avoid timeouts by running the import through Ajax, and resuming them (with a new request) whenever we get close to the timeout limit.

[melchoyce]

For some design reference, the Calypso importer is pretty nice: https://cloudup.com/cWNtcPQok6U

[rmccue]

@melchoyce See #30 :)

[melchoyce]

Sorry, missed that

[rmccue]

Sorted in #26!