Make the License more explicit

[stadler]

Trema is licensed under MIT License.
This can be seen in the pom.xml license section.
We should state this also in the documentation, because potential users may not directly look into the pom file to check if it is open source.

Also currently the Footer of the Documentation currently says:
"Copyright © 2014–2015 Netcetera. All rights reserved."
Maybe we should use something more friendly.

[marcelstoer]

There's a LICENSE file in the root of each repository... I just added a license badge to the README.md in each of the four repositories.