Plug various leaks of capability references.

In all cases, the capabilities would eventually be garbage collected, but if a user explicitly called .close() in hopes of dropping a reference proactively, it could be ignored because a leaked reference still existed.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "capnp",
-  "version": "0.1.4",
+  "version": "0.1.5",
   "description": "A wrapper for the C++ Cap'n Proto library",
   "keywords": [ "capnproto", "serialization" ],
   "homepage": "http://capnproto.org",