jsnext support

PR #30:

Fixes #25 - logs

• relied on debug() to log locator warnings
• moved logic to dedupe duplicates into its own method for better testing

Fixes #27 - method deprecation:

* listAllResources() -> getAllResources()
* listBundleNames() -> getBundleNames()

PR #29:

Fixes #25 - support rulesets generation thru rulesetFn configuration

This will allow us to do something like:

var locator = new Locator({
    rulesetFn: function (bundle) {
        if (bundle.pkg && bundle.pkg['jsnext:main']) {
            return { /* create synthetic rules for ES6 packages */ };
        }
    }
});

Fixes filter behavior by allowing falsy filter value and empty object {} as filters that will pass for all files and resources, but fails if there is a key in the filter object that doesn't match extensions or types. You can now do:

locator.getBundleResources('foo'); // return all resources in bundle
locator.getBundleResources('foo', {}); // return all resources in bundle
locator.getBundleResources('foo', { crazy: 1 }); // return no resources
locator.getBundleResources('foo', { types: 'model' }); // return model resources