Added UAT sandbox support
slight refactoring of Sandbox flag

Instead of using a variable within the module and exposing a getter and setter, let's simplify things by making sandbox a property of the module, so that it can be get and set directly.  Any time a URL needs to be generated, we'll evaluate the sandbox property.
update README for Sandbox flag
Merge pull request #6 from Dwolla/master

Merge? :)