Fixes #31
Add the switch sandbox_response in order to mock a return response from Paypal.
No server hit is done when the configuration is set.
Fixes #30. Validate Paypal response properly
fsockopen can lead to chunked response. This can lead to read incorrectly Paypal's response.
Changed fsockopen for curl.
Introducing proxy in order to talk to Paypal.
Add support for Payment Data Transfer (PDT).
Missing variable
Merge branch 'feature-pdt'
Merge pull request #32 from jsgoupil/master

Fixes #31