Index installed sounds and implement ARI sounds queries

This adds support for stasis/sounds and stasis/sounds/{ID} queries via
the Asterisk RESTful Interface (ARI, formerly Stasis-HTTP).

The following changes have been made to accomplish this:
* A modular indexer was created for local media.
* A new function to get an ast_format associated with a file extension
  was added.  
* Modifications were made to the built-in HTTP server so that URI
  decoding could be deferred to the URI handler when necessary.
* The Stasis-HTTP sounds JSON documentation was modified to handle
  cases where multiple languages are installed in different formats.
* Register and Unregister events for formats were added to the system
  topic.

(closes issue ASTERISK-21584)
(closes issue ASTERISK-21585)
Review: https://reviewboard.asterisk.org/r/2507/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@392700 65c4cc65-6c06-0410-ace0-fbb531ad65f3

rest-api/api-docs/sounds.json

@@ -58,6 +58,19 @@
 		}
 	],
 	"models": {
+		"FormatLangPair": {
+			"id": "FormatLangPair",
+			"properties": {
+				"language": {
+					"required": true,
+					"type": "string"
+				},
+				"format": {
+					"required": true,
+					"type": "string"
+				}
+			}
+		},
 		"Sound": {
 			"id": "Sound",
 			"properties": {
@@ -71,13 +84,10 @@
 					"description": "Text description of the sound, usually the words spoken.",
 					"type": "string"
 				},
-				"lang": {
-					"required": true,
-					"type": "string"
-				},
 				"formats": {
 					"required": true,
-					"type": "List[string]"
+					"description": "The formats and languages in which this sound is available.",
+					"type": "List[FormatLangPair]"
 				}
 			}
 		}