Add license header to all source files

[bonimy]

License headers are a standard practice with most licenses. While the MIT license (used with this project), does not require this, I'd like to still see it there.

Proposed header format:

// <copyright file="%FileName%" company="R3 Productions">
//     Copyright (c) 2008 Roice Nelson, Nelson Garcia. All rights reserved. Licensed under
//     MIT License. See LICENSE in project root for full license information, or visit
//     https://opensource.org/licenses/MIT
// </copyright>