Removed duplicate files.
Removed extraneous calls to `super`

Calling `super` with no arguments in Ruby automatically passes the
arguments that were given to the current implementation along as well.
This was causing errors when the superclass's initialize signature was
different (mostly the child classes are accepting attribute hashes and the
parent classes expect no arguments).
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
Fix dependency requiring
Added a facade for simplified services usage
Move TaxCloud::Base functionality into TaxCloud

basically making it a singleton.

Replaced the initializer with a configuration struct.
Make CartItemTax#amount a float
Merge pull request #1 from Scoutmob/master

Pulling in updates from Scoutmob/tax_cloud