Make es6-promise a peer dependency so that users don't end up with two copies of the promise library and instanceof works correctly.
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.
Allow Javascript servers to detect when all clients have closed a capability.
Bump version.
Fix readme.