Add GET /tables/:name/keys.

Added an endpoint for retrieving all keys on a table:

```
$ curl localhost:8585/tables/foo/keys
["bob", "susy", ...]
```

I thought about making `/tables/foo/objects` the endpoint but I didn't want people accidentally hitting it while navigating the URL. Also, `/objects` implies the full object which would end up being a dump of the whole table.

/cc: @boourns @orenmazor @snormore