Relicense SHOGun to BSD-3-clause

In #55 we agreed upon relicensing SHOGun to BSD-3-clause: http://opensource.org/licenses/BSD-3-Clause.

This pull request

* Renames the `LICENSE.txt` to `LICENCE.md`
* Adds the BSD-License of that file to contain the BSD-3-clause license
* Adds a license header to all source files with the text of the BSD-license
* Changes the `pom.xml` to correctly reference the BSD-license.

All contributors agreed upon relicensing in issue #55.

Please review.

Closes #55.