Automatically close capstone reference when GC'd

[parasyte]

This work has started in a new branch: https://github.com/parasyte/node-capstone/tree/weak

I tested it with mocha using the --expose-gc argument, and close was indeed called. I didn't include the mocha tests in the commit because I'm not familiar with it, and not sure I want to rewrite all of the tests to use a different assertion library.

I couldn't get mocha's diff feature to work as well as diffMatcher, so it would be some extra work to support.