Add a fallback if pushState is unavailable

[prollynomial]

Currently Cartier will break if pushState is unavailable - a fallback should exist.
Options:

• Fall back to hash prefixed URLs
  • makes URLs ugly, but won't require page reloads
• Fall back to window.location.href
  • shouldn't be as much of an issue for perma-cached single-page apps because the only time the assets actually get downloaded is when the hash of the scripts or styles changes (e.g. Application-ecd6d19.js)