Implement very basic PayPal API wrapper

Summary:
Ref T2787. The [[ https://github.com/paypal/merchant-sdk-php | official PayPal PHP SDK ]]:

  - ...is 27,000+ lines long!
  - ...is all generated code!
  - ...contains a non-ironic `PayPalAPIInterfaceServiceService` class as its primary interface!
  - ...uses SOAP!
  - ...is completely broken (literally missing most of the classes it needs to run)!

Instead, provide a minimal wrapper for the guts. The actual API is reasonable and there's a name-value pair flavor available.

I'm using this to implement the "Express Checkout" workflow in Phortune.

Test Plan: Made PayPal API calls using the test script.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D5808