Consistent APIs: Get field mapping API includes 'mappings'

The get field mapping API now includes a mappings element after the index in its JSON

Added more consistent endpoint /{index}/_mapping/{type}/field/{fields}
and added endpoint /_mapping/{type}/field/{fields}
which are also used in tests

Added rest spec tests for wildcards and _all

Relates #4071

NOTE: This is not yet complete for 1.0. We need to return an empty JSON document instead
of a 404 if the field of an existing index and type is not found. However this is not
possible with the current data structure being returned. Needs to be finished for 1.0.