Merge stored/index fields into _formatted

The goal of this was to close #1766 in which `_timestamp` was not working. The reason `_timestamp` doesn't work is because it is never returned, unless it is `store: true` and you explicitly ask for it. The pull does the following to resolve the issue:

- Always requests all stored fields
- Merges stored field values into _formatted for display in discover. This is also useful for scripted fields. (#1537)
- Breaks out the field mapper for testing
- Overrides the mapping of `_timestamp` to be `date`, since it always is and must be. This makes sure it is formatted as a date and is available for date_histogram aggs.

![screen shot 2014-11-20 at 4 18 38 pm](https://cloud.githubusercontent.com/assets/1250387/5135029/1c8a7e44-70d1-11e4-936c-3075831fda31.png)