Handle merchantCustomerId in CreateCustomer method

1. Add a new overloaded CreateCustomer method to take the
merchantCustomerId.  Thanks to the good pull request from dgw2jr
https://github.com/AuthorizeNet/sdk-dotnet/pull/27

2. CIM\Customer.cs -  in Customer object constructor, remove
auto-generated GUID for ID and add "MerchantCustomerID" instead.