loadUrl enhancements (#134)

`loadUrl` enhancements

- Make `options` parameter optional
- Allow partial overriding of instance options when calling `loadUrl` directly
- Make `requestOptions` optional
- Document `loadUrl` usage and provide examples

tests/lib/util/extend.js

@@ -0,0 +1,16 @@
+var tape = require("tape")
+
+var extend = require("../../../lib/util/extend")
+
+tape("test extend method", function(t) {
+  var obj = {one: 1, two: 2}
+  var extended = extend({}, obj, {two: "two", three: 3})
+
+  t.notEqual(obj, extended, "extended object isn't the original object")
+
+  t.notSame(obj, extended, "extended object doesn't have the same values as original object")
+
+  t.notSame(obj.two, extended.two, "extended object value overwrites value from original object")
+
+  t.end()
+})