Scripted fields in index pattern

Adds scripted fields to the index pattern

Part of the first couple tasks on #1537 

- Add scriptable property on field param
- Mock scritpedFields property to index-pattern
	- currently always just an empty array
- Add tabbed field interface for viewing scripted fields
- Code cleanup
	- use `self` in courier, saved_object, index_patterns, _mapper and _index_pattern
	- fix inheritence pattern in doc_source
	- remove unused (and broken) `getFieldsFor` method in courier