Added PerfNow.js

*A 116 byte window.performance.now high resolution timer polyfill with Date fallback.*

Tested Environments
-------------------
- Mac OS X
	- Google Chrome
	- Safari
	- Firefox
	- Opera

- Windows
	- Google Chrome
	- Safari
	- Firefox
	- Opera
	- Internet Explorer

How to use
----------

```javascript
var t0 = performance.now();
doSomething();
var t1 = performance.now();
console.log("Call to doSomething took " + (t1 - t0) + " milliseconds.")
```
[https://github.com/daniellmb/perfnow.js](https://github.com/daniellmb/perfnow.js)

Why?
----
Unlike other timing data available to JavaScript (for example `Date.now`), the timestamps returned by `Performance.now()` are not limited to one-millisecond resolution. Instead, they represent times as floating-point numbers with up to microsecond precision.

Also unlike `Date.now`, the values returned by `Performance.now()` always increase at a constant rate, independent of the system clock which might be adjusted manually or skewed by software such as the Network Time Protocol.

When?
-----
There are a many situations where you should use this high resolution timer instead of a basic timestamp:

- **benchmarking**
- calculating framerate with precision
- cueing actions or audio to occur at specific points in an animation or other time-based sequence
- game or animation runloop code