[BUGFIX] Fix bug with styling same element more than once.

I came across an odd bug while using this in Ember.js. My Ember.js component would remove my `select` box and put in back into the DOM quite frequently. When it put it back into the DOM, it obviously had to be styled again. However, I used the same ID for the node every time, and EasyDropDown uses the DOM ID to detect if it's already been styled. Because of this, EasyDropDown thought my element was already styled, even though it had been deleted and re-created since it styled it last. I added a small flag on the DOM node to detect whether the item is truly styled or not. If it's not, it now overwrites the instance held in the `instances` map.

It's a small use case, but an important one if you're using something like Ember.js that dynamically adds/removes the element without destroying it properly. There aren't any tests, so I wasn't able to see for sure if my change broke anything. However, it's working fine for me so far, and it seems like a somewhat isolated change. If there are any regressions, I'd be happy to work through them.