add withCredentials to base methods

[johnryan-wf]

This adds withCredentials as an optional parameter for BaseClient requests. When not using BrowserClient, this flag is ignored. Defaults to false.

@nex3 Please let me know if you would prefer a different approach

[johnryan-wf]

this addresses #5 I think

[nex3]

I don't want to add anything platform-specific to the shared Request class. Ultimately, my goal is to move http to a more shelf-like API with a context getter, which would work well for this, but that's a ways away. For now, it's probably best to put this property on BrowserClient.

[johnryan-wf]

@nex3 I agree that's a cleaner solution. Done.