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
Remarkup: Allow Odd Line Breaks Before Blocks

Summary: If there are two or more consecutive line breaks (i.e. at least one empty line), the first non-empty line should be considered the first line of the block. Currently, that is not the case if there are an odd number of line breaks because the last empty line will be considered the first line of the block. This breaks things like HTML tables (which assumes the block starts with `<table>`), code blocks (which special cases the first line if it is `lang=something`), etc.

Test Plan: Ran unit tests successfully.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: leebyron, aran, Korvin, keito

Differential Revision: https://secure.phabricator.com/D5814