Add deprecated WC_Paypal class

WC_Paypal was removed in 2.0 in favour of WC_Gateway_Paypal, this
patch deprecates the original WC_Paypal class & returns an instance
of the WC_Gateway_Paypal object instead of potentially causing a
fatal error.
Merge pull request #2432 from thenbrent/deprecated-paypal-class

Add deprecated WC_Paypal class