Tweaked Gemfile to work with newer versions of bundler.

Removed explicit dependencies on bundler and jeweler as they were
causing 'bundle install' to break.
Added support for Ebay Sandbox urls.

Added support for using Ebay Sandbox urls (through a configuration
variable).

Changed the way Finding and Shopping generate their base urls. Instead
of a class variable, they now call self.base_url, which reads the
sandbox configuration variable and either creates a sandbox url or a
production url.
Converted BASE_URL_PREFIX,SUFFIX to class methods.
Fixed class vs. instance method issue on Shopping
Updated Shopping#find_products to version 793.
Fixed Ebay Url argument typos.
Updated the Shopping Api to version 793.

Updated the conditions under which the Shopping Api calls raise
ArgumentErrors, involved changing the cases of the parameters (they
changed in the Ebay Api) and adding support for specifying either a
ProductID or a ProductID.type, ProductID.Value pair for searching by
ProductIDs other than Ebay ReferenceIDs.
Merge branch 'master' of https://github.com/yarmiganosca/rebay into yarmiganosca-master

Conflicts:
	Gemfile
	Gemfile.lock
	lib/rebay/api.rb
	lib/rebay/shopping.rb
	spec/api_spec.rb
	spec/shopping_spec.rb