Clj 1.3 custom channel factory

Hello Zach,

I just patched aleph to add a way to customize the underlying channel factory used by netty/create-client. In my case, I would like to choose the number of "bosses".

With this patch, we can either set the default channel factory or passe it as an option when creating the client.

All tests still pass. Let me know what do you think about this.

David