promisify - converts node callbacks to promises

Just starting a conversation here, I was talking to Yehuda about this tonight on freenode.

Pretty simple, you can turn the usual node callback pattern into promises like so:

```javascript
var fs = require('fs');
var RSVP = require('rsvp');

// create a new function that'll return a promise
var readFile = RSVP.promisify(fs.readFile);

// usage
readFile('sample.txt').then(function(data) {
  console.log(data.toString());
});
```

I didn't know where to put tests in this repository since it just runs the Promises/A specs. Before its merged it would need some test coverage and a better commit message, but anyway ...

Asking for your thoughts on the validity of adding this to rsvp and if this is the right API?