Cleaned up response status checking.

Added ok?, error?, warning?, and informational? methods to ResponseBase
and ResponseMessage
Update ResponseBase#messages

to return ResponseMessage instances per the comments
Replace GetTICsByGroupResponse

with GetTICsResponse. They were identical, so there's no reason to keep
both around.
Simplify message handling code in ResponseBase

I found an easier to read way to handle messages being either an array
or a single message.
Removed unnecessary initializers

from response objects without values.
Cleaned up the response from the lookup service.

CartItemResponse wasn't even being used, and CartItemsResponse wasn't
really necessary.

Created a CartItemTax model to do the job that I think CartItemResponse
was originally intended for, and put it alongside of the other data
structures in lib/tax_cloud.
Made the VerifyAddressResponse consitent

with the rest of the responses, including inheriting from ResponseBase,
and gave it the same response codes.
Made the TIC service responses more idiomatic