Switch to MIT license (same as React)

[Daniel15]

Switches from the BSD + Patents license to the MIT license, to match React.

Most replacements were done with a regex in Notepad++
Find:

/\*\s*\r?\n \*  Copyright \(c\) 201\d(\-Present)?, Facebook, Inc\.\s*\r?\n \*  All rights reserved.\s*\r?\n \*\s*\r?\n \*  This source code is licensed under the BSD-style license found in the\s*\r?\n \*  LICENSE file in the root directory of this source tree. An additional grant\s*\r?\n \*  of patent rights can be found in the PATENTS file in the same directory.\s*\r?\n \*/

Replace with:

/**\r\n * Copyright \(c\) Facebook, Inc. and its affiliates.\r\n *\r\n * This source code is licensed under the MIT license found in the\r\n * LICENSE file in the root directory of this source tree.\r\n */

Fixes #524