Create bigcommerce.php

Single package file. 
Updated with 11/10/2014 fixes by @bc-jz.
https://github.com/bigcommerce/bigcommerce-api-php/pull/109
-Adds tax class to list tax resource
-Fixes various typos in Shipment and Sku classes.
-Fixes deadlocks in curl put function, as originally reported by: 
--- http://www.jasperstudios.com/bigcommerce-api-code-patch/