Proxy temporary and saved dashboards to ES

Allow requests to save and fetch dashboards from Kibana's internal index on
Elasticsearch. I believe they need the same HTTP methods as other indexes.

For temporary dashboards requests look like:

    127.0.0.1 - - [13/Aug/2014 13:55:56] "POST /kibana/kibana-int/temp?ttl=30d HTTP/1.1" 201 92 0.1674
    127.0.0.1 - - [13/Aug/2014 13:56:00] "GET /kibana/kibana-int/temp/GjO0MfT5QL6dLqOytd6qDw?1407934560237 HTTP/1.1" 200 3634 0.1673

For permanent dashboards requests look like this:

    127.0.0.1 - - [13/Aug/2014 14:00:09] "PUT /kibana/kibana-int/dashboard/example HTTP/1.1" 400 195 0.168

Although right now I get a 400 response and an error from our Elasticsearch
that I don't think is related to kibana-rack:

    MapperParsingException[failed to parse, document is empty]

---

Does that look right to you?
Great! If you want, you can leave it open and just throw the `WIP` label on it, and change the label to `Ready` when you're done. That way it can keep building on CI whenever you push :smiley: