box-sizing: content-box (Issue #75)

Assume default `box-sizing: content-box` for hopscotch bubble and child elements, unless overridden by a later selector. This allows Hopscotch to play nicely with Bootstrap 3 and any other CSS frameworks or resets that may apply `box-sizing: border-box` to the whole page.

Tested locally using `/demo` page by getting latest version of Bootstrap 3 and replacing Bootstrap 2 with it.