ARI: Add subscription support

This patch adds an /applications API to ARI, allowing explicit management of
Stasis applications.

 * GET /applications - list current applications
 * GET /applications/{applicationName} - get details of a specific application
 * POST /applications/{applicationName}/subscription - explicitly subscribe to
   a channel, bridge or endpoint
 * DELETE /applications/{applicationName}/subscription - explicitly unsubscribe
   from a channel, bridge or endpoint

Subscriptions work by a reference counting mechanism: if you subscript to an
event source X number of times, you must unsubscribe X number of times to stop
receiveing events for that event source.

Review: https://reviewboard.asterisk.org/r/2862

(issue ASTERISK-22451)
Reported by: Matt Jordan
........

Merged revisions 400522 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@400523 65c4cc65-6c06-0410-ace0-fbb531ad65f3

rest-api/api-docs/endpoints.json

@@ -13,7 +13,7 @@
 			"operations": [
 				{
 					"httpMethod": "GET",
-					"summary": "List all endoints.",
+					"summary": "List all endpoints.",
 					"nickname": "getEndpoints",
 					"responseClass": "List[Endpoint]"
 				}

rest-api/api-docs/events.json

@@ -293,6 +293,11 @@
 					"required": true,
 					"type": "Channel",
 					"description": "The channel that signaled the user event."
+				},
+				"userevent": {
+					"required": true,
+					"type": "object",
+					"description": "Custom Userevent data"
 				}
 			}
 		},
@@ -338,6 +343,17 @@
 				}
 			}
 		},
+		"EndpointStateChange": {
+			"id": "EndpointStateChange",
+			"extends": "Event",
+			"description": "Endpoint state changed.",
+			"properties": {
+				"endpoint": {
+					"required": true,
+					"type": "Endpoint"
+				}
+			}
+		},
 		"StasisEnd": {
 			"id": "StasisEnd",
 			"extends": "Event",