feature(instruments): Add a function to automatically time arbitrary functions

The immediate use case I have in mind is exporting functions from modules to always be timed when they are called.

```javascript
var timeFunction = require('rackspace-shared-utils/lib/instruments').timeFunction;

function expensiveFunction(foo, bar, callback) {
  ... do some stuff ...
  callback();
}

exports.expensiveFunction = timeFunction(expensiveFunction);
```