Allow Subscriptions to be created by clients.

Summary: This will allow downstream dependencies to make use of the Subscription type.

Reviewers: O2 Material Motion, #material_motion, O4 Material Apple platform reviewers, chuga

Reviewed By: O4 Material Apple platform reviewers, chuga

Tags: #material_motion

Differential Revision: http://codereview.cc/D2271

src/IndefiniteObservable.swift

@@ -86,7 +86,7 @@ public final class Subscription {
     unsubscribe()
   }
 
-  init(_ disconnect: @escaping () -> Void) {
+  public init(_ disconnect: @escaping () -> Void) {
     self.disconnect = disconnect
   }