Callback wrapping utility

## USAGE

```javascript
var wrappy = require("wrappy")

// var wrapper = wrappy(wrapperFunction)

// make sure a cb is called only once
// See also: http://npm.im/once for this specific use case
var once = wrappy(function (cb) {
  var called = false
  return function () {
    if (called) return
    called = true
    return cb.apply(this, arguments)
  }
})

function printBoo () {
  console.log('boo')
}
// has some rando property
printBoo.iAmBooPrinter = true

var onlyPrintOnce = once(printBoo)

onlyPrintOnce() // prints 'boo'
onlyPrintOnce() // does nothing

// random property is retained!
assert.equal(onlyPrintOnce.iAmBooPrinter, true)
```
