URLFilter stream filters only updates with URLs

 On branch url_filter

	new file:   lib/url_filter.js
	new file:   test/fixtures/twitter/update_with_url.json
	new file:   test/fixtures/twitter/update_without_url.json
	deleted:    test/smoke_test.js
	new file:   test/url_filter_test.js
Merge pull request #3 from kixxauth/url_filter

URLFilter stream filters only updates with URLs