Various improvements

Hey guys,

I refactored a few methods and added the ability to automatically handle DOM elements added with JavaScript (after window.onLoad).

I also replaced the class `buffer-share` with the data attribute `data-buffer-share`, it's more consistent in my opinion.

```
<a data-buffer-share
data-url='http://blog.bufferapp.com' 
data-text="The Buffer Blog!" 
data-preferred-login="twitter" 
data-partner-source='feedly' 
data-partner-placement='mini' 
data-via='sunils34'>Share to Buffer</a>
```

Feedback are welcome!