issue #1501: allow Closeable, top-level apis to be used as opposed to RestContext
issue #1501: add Closeable annotation to all top-level apis
added temporary workaround for AutoCloseable which will be deleted in jclouds 1.7
Merge pull request #1503 from jclouds/closeable-api

expose Closeable, top-level apis via ContextBuilder.buildApi()