Wrap Listener instances in WeakRef

[jonathanhefner]

The @listeners variable holds a reference to each Listener instance created by Listen.to and can only be cleared by calling Listen.stop. This can prevent Listener instances from being garbage collected if they are abandoned before Listen.stop is called.

This commit wraps Listener instances in WeakRef before adding them to @listeners, to allow them to be garbage collected.

Partially addresses #476.

[coveralls]

Coverage increased (+0.1%) to 97.831% when pulling 63eff89 on jonathanhefner:listeners-memory-leak into ba5059c on guard:master.

[coveralls]

Coverage increased (+0.03%) to 97.757% when pulling 694e28b on jonathanhefner:listeners-memory-leak into ba5059c on guard:master.

[jonathanhefner]

@headius Does this look all right to you? I decided to forgo clearing dead WeakRefs in Listen.to because it would require using Mutex instead of a simple Queue to stay thread-safe. I can make that change if you prefer. However, if someone were to create so many listeners that dead WeakRefs could become a problem, they might prefer using Listener.new instead (since it will have less overhead).