Consistent REST API changes for GETting data

* Made GET mappings consistent, supporting
  * /{index}/_mappings/{type}
  * /{index}/_mapping/{type}
  * /_mapping/{type}
  * Added "mappings" in the JSON response to align it with other responses
* Made GET warmers consistent, support /{index}/_warmers/{type} and /_warmer, /_warner/{name}
  as well as wildcards and _all notation
* Made GET aliases consistent, support /{index}/_aliases/{name} and /_alias, /_aliases/{name}
  as well as wildcards and _all notation
* Made GET settings consistent, added /{index}/_setting/{name}, /_settings/{name}
  as well as supportings wildcards in settings name
* Returning empty JSON instead of a 404, if a specific warmer/
  setting/alias/type is missing
* Added a ton of spec tests for all of the above
* Added a couple of more integration tests for several features

Relates #4071