make a better fault-tolerant approach to handle un-enhancing page if script is blocked or doesn't load

[scottjehl]

[ssirois]

Merge of #16 introduces a ReferenceError: docClasses is not defined since docClasses is indeed, never defined before being used inside addEnhanceClass and removeEnhanceClass

It feels like htmlClasses should have been used instead and that line 158 should be remove in favour of line 174.

I'll go on and make a pull request after making sure my hypothesis is correct.

Would you like me to open a new issue stating this problem or is it fine to continue with this current issue? Since there is no description orienting my thoughts on where this particular issue is going, I'm not sure if a new issue is necessary or not.