Fix #26: allow nested elements

Change the regexp in `isWS()` to only match space characters outside of element tags. The space in `<div class="foo">` is not matched, but the spaces in `<div>This is inner content</div>` are matched.

To make this more efficient, potential breakpoint indices could be held in an array outside of isWS.

A working demo can be seen [here](http://codepen.io/rileyjshaw/pen/oggBxz).
I think the results of the INLINE CONTENT TEST are not good because the content is not representative of normal content since the images are too wide. Inline images are more likely to be icon size, right? I think widths should be between 20px-60px.