Add AMD support issue #10

This pull request adds AMD support (http://requirejs.org/docs/whyamd.html) issue #10 .

Since the library exposes two functions, in AMD context an object is used to wrap around them. Example usage would be:

```js
require(["sprintf/sprintf"], function(Sprintf) {
        console.log(Sprintf.sprintf("%2$s %3$s a %1$s", "cracker", "Polly", "wants"));
        console.log(Sprintf.vsprintf("The first 4 letters of the english alphabet are: %s, %s, %s and %s", ["a", "b", "c", "d"]));
});
```