Added endpoint for basket/applyvoucher
( http://api.7digital.com/1.2/static/documentation/7digitalpublicapi.html#basket/applyVoucher )
can't do integration tests without a supply of valid voucher codes, so will test in a client (the website)
Merge pull request #95 from AnthonySteele/ApplyVoucherEndpoint

Added endpoint for basket/applyvoucher