[helpers] Add helper function to make custom instrumentation easier

This adds a helper function to simplify the process of writing custom instrumentation. The flow will now look something like this:

```
var tv = require('traceview')
var fs = require('fs')
 
var readFile = fs.readFile
fs.readFile = function (path) {
  var callback = arguments[arguments.length - 1]
  var run = arguments.length > 2
    ? readFile.bind(this, path, arguments[1])
    : readFile.bind(this, path)
  
  return tv.instrument(run, callback, function (last) {
    return last.descend('fs', {
      Method: 'readFile'
    })
  })
}
```

Next up is writing docs on using this new interface.