Instance local caching for "enabled" property (no-leak)

[thewalkingmonty]

I initially overlooked @Qix- fix #740 to issue #678 and came up with almost the same solution, with just the difference the enabled property is managed in a slight different way that:

• caches the status of the enabled property maintaining a stable performance when used in non dynamic context
• do not change the way the enable function interacts with the enabled property

So I'm proposing my fix besides @Qix-'s one as it may add some little value.

[coveralls]

Coverage increased (+0.2%) to 89.568% when pulling c3646d0 on thewalkingmonty:no-leak into 6087241 on visionmedia:no-leak.