Update license headers BSD+Patents -> MIT

Did find and replace in TextMate.

```
find: (?:( \*)( ))?Copyright (?:\(c\) )?(\d{4})\b.+Facebook[\s\S]+(?:this source tree|the same directory)\.$
replace: $1$2Copyright (c) $3-present, Facebook, Inc.\n$1\n$1$2This source code is licensed under the MIT license found in the\n$1$2LICENSE file in the root directory of this source tree.
```