Add observer to add customer to guest order by matching email (Fixes #19)

[steverobbins]

No description provided.

[srenon]

@steverobbins .... will a customer be created automatically for every guest order?

[steverobbins]

@srenon No. If an order is created through guest checkout, but the email matches the a customer account that already exists, this will add the guest order to their account.

[srenon]

@steverobbins .... Sorry for the confusion earlier—I only took a quick look at the code and which observer was being used.

This is actually a great feature. That said, I think it would be beneficial to add a system configuration option that allows this behavior to be enabled or disabled, giving more flexibility depending on the use case.