Using url of Model or Collection properly while constructing globalName in ioBind function

Both Backbone's Model and Collection accept url attribute in either a string or a function. However, ioBind function defined in model.js and collection.js use different invoke method to construct globalName. And in this case, it restricts developer has to define url as a function in Model, and define url as a string in Collection only.

This commit is to solve this problem. Hope it be helpful to backbone.iobind project. Thanks for your great work to create such wonderful package!