Add config object to FiddlerProxy plugin to be able to set Fiddler URL

[aleksanderson]

Provided changes to FiddlerProxy plugin might be helpful when using Fiddler not on the local machine. For example, developing on mac, you might want to use fiddler on the windows machine running inside a VM like VMWare Fusion or Parallels Desktop. (Telerik provided instructions on how to set up such workflow: http://goo.gl/pdgQBa).

Assuming the current PR the plugin call signature would be the following:

//default fiddler url, e.g http://127.0.0.1:8888
client.plugins = [FiddlerProxy()];

//custom fiddler url
client.plugins = [FiddlerProxy({ url: 'http://192.168.1.37:8888' })];

[zetlen]

Thanks, @aleksanderson, for your patience and for the PR. I had to make some minor changes to get the new tests to pass and to make your new plugin API tolerate an existing config object with a missing URI, but otherwise the code is unmodified.

Z