Add spec for ext_url filter, refactor

spec/javascripts/unit/darkswarm/filters/ext_url_spec.js.coffee

@@ -0,0 +1,19 @@
+describe "ensuring absolute URL", ->
+  filter = null
+
+  beforeEach ->
+    module 'Darkswarm'
+    inject ($filter) ->
+      filter = $filter 'ext_url'
+
+  it "returns null when no URL given", ->
+    expect(filter(null, "http://")).toBeNull()
+
+  it "returns the URL as-is for http URLs", ->
+    expect(filter("http://example.com", "http://")).toEqual "http://example.com"
+
+  it "returns the URL as-is for https URLs", ->
+    expect(filter("https://example.com", "https://")).toEqual "https://example.com"
+
+  it "returns with URL with prefix added when a relative URL is given", ->
+    expect(filter("example.com", "http://")).toEqual "http://example.com"