Fix polyfill methods being removed in Safari 8

We ran into an interesting issue with usertiming.js in the new Safari 8:

Ssince there's Navigation Timing support but no Performance Timeline support, the `window.performance` object is provided by the browser, but we still need to modify it to add methods like `performance.mark` and `performance.measure`. Safari 8 doesn't support adding properties to the built-in performance object in a stable way though. Triggering certain events or opening/closing the dev tools causes the object to be reset, so all of the properties we've added with the polyfill disappear. (See this [JS Fiddle](http://jsfiddle.net/dss8wudw/3/) in Safari 8 to test it out.)

Not sure if the solution here is necessarily the best way to solve this, just wanted to toss it out there and see what you think.

This PR creates a new proxy object that we attach all the shims to. If we needed to shim anything, we set `window.performance` to the new object, copying over references to any built-in performance methods/properties onto the new object so those are also available. It's been tested in IE 9-11, Safari 7-8, latest Firefox & Chrome — if you're interested in going this route I can also try to get my hands on VMs of the earlier IE versions and test there too.