Change to MIT

Open Issues

• How is the copyright holder? ("Heiko Bornholdt and Kevin Röbert" vs. "The drasyl project")
• Update copyright header in existing files
• Full license header or just point to LICENSE?

Copyright (c) <year>, <copyright holder>
All rights reserved.

This source code is licensed under the MIT license found in the
LICENSE file in the root directory of this source tree. 

Closes #143 (closed)