Stash checksum and/or timestamp in Redis

Similar to #25, it would be fantastic if ember-deploy-redis would also stash a checksum (or the commit SHA) and/or a build date/time (or the commit timestamp) of the index to use in the the `ETag` and `Last-Modified` HTTP headers. Something like:

```
foo:index:current => '...'
foo:index:current:meta => checksum '787b3dd284fd99202adfa4e18a28126d159552c5'
timestamp 'Wed Sep 16 19:28:37 2015 +0200'
```

And then...

``` ruby
require 'sinatra'
require 'redis'

get '/' do
content_type 'text/html'
expires 3_600, :public

redis = Redis.new
index_key = redis.get("foo:current")
index_key = "foo:#{params[:index_key]}" if params[:index_key]

meta = redis.hgetall("#{index_key}:meta")
etag meta[:checksum]
last_modified meta[:timestamp]

redis.get(index_key)
end
```

Stashing the content in the same hash as the metadata would be ideal (to limit the Redis hits) but would break backwards compatibility.