Update PHP to allow caller to set curl options

[tzs]

Many production environments restrict outgoing network access on their servers that deal with payments, only allowing such access through proxies that vet the destinations before allowing access. This patch provides an option for the caller of verifyIPN() to provide the curl object for verifyIPN() to use instead of allocating its own curl object. This allows the caller to set any proxy options, or other options, that may be required in order to allow the connection back to Paypal to work.

[tzs]

phpDocumentor comments added. The description for __construct seems awkward, but I could not think of anything better.