fixes #25: open up the possibility to generate rulesets dynamically

This will allow us to do something like:

```javascript
var locator = new Locator({
    rulesetFn: function (bundle) {
        if (bundle.pkg && bundle.pkg['jsnext:main']) {
            return { /* create synthetic rules for ES6 packages */ };
        }
    }
});
```

tl;dr: we can read package.json for a package, and generate rules the way we want it based on the package information without having to have a designated ruleset.
yes, that's correct, saying it is default ruleset computation at a particular folder.

I guess we can consider `DEFAULT_RULESET` as a name with a meaning rather than the actual rules.