Improved License Header

[Adlai-Holler]

The license headers in the repo are a bit of an eyesore (16 lines for ASDisplayNode.h), and keeping them correct when making PRs can be a real pain e.g. #743.

I propose that we standardize on this pattern, indefinitely:

//
//  ASDisplayNode.h
//  Texture
//
//  This file is subject to the copyright, license, and patent terms
//  in the LICENSE file and PATENTS files at the root of this source tree.

instead of the current

//
//  ASDisplayNode.h
//  Texture
//
//  Copyright (c) 2014-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under the BSD-style license found in the
//  LICENSE file in the /ASDK-Licenses directory of this source tree. An additional
//  grant of patent rights can be found in the PATENTS file in the same directory.
//
//  Modifications to this file made after 4/13/2017 are: Copyright (c) 2017-present,
//  Pinterest, Inc.  Licensed under the Apache License, Version 2.0 (the "License");
//  you may not use this file except in compliance with the License.
//  You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//

Can we do that?

[Adlai-Holler]

cc @nguyenhuy @maicki @garrettmoon @appleguy

[nguyenhuy]

I'm down.

@garrettmoon Maybe we should ask Legal?