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?
How about a constant named `PROXY_REGEX` up at the top of the class with something like (will be useful to do that too if the proxy routes keep growing :smile:):

```ruby
PROXY_REGEX = %r{\A(
  (
    (
      (/_(aliases|nodes)) |
      (.+/_(aliases|mapping|search)) |
      (/#{settings.kibana_index}/(temp|dashboard))
    )
    (/.+)?
  )
)}x
```

Could you add some examples like `/kibana-int/temp`, etc. to the proxy test [here](https://github.com/tabolario/kibana-rack/blob/master/spec/unit/kibana/rack/web_spec.rb#L64)? I have a generic test there that you can just add additional endpoints to verify.