Clarify the license and copyright terms of the project

To make clear the license and copyright of the project and its contributions the LICENSE now has (1) copyright (2) license and (3) contributor agreement sections. These explicitly declare the project-wide BSD-2 license for all contributions, assign copyright to the respective contributors of each contribution as detailed by versioning, and state that all contributions agree to these license and copyright terms.

Since only the versioning history holds a perfect record for attribution and copyright, file headers and the like are deprecated.

Nothing material has changed in the licensing and copyright terms already in place. This is merely a clarification.