Returns whether Unpoly supports the current browser.
If this returns false Unpoly will prevent itself from booting
and ignores all registered event handlers and compilers.
This leaves you with a classic server-side application.
This is usually a better fallback than loading incompatible Javascript and causing
many errors on load.
Graceful degradation
This function also returns true if Unpoly only support some features, but can degrade
gracefully for other features. E.g. Internet Explorer 9 is almost fully supported, but due to
its lack of history.pushState
Unpoly falls back to a full page load when asked to manipulate history.
