fix #4: Coding standards

- added some missing attributes to the project formatter
- added maven-checkstyle-plugin to verify the code against our standards
- added develop/checkstyle/google-checks.xml as checkstyle definition
- formatting of sourcecode applied
- added a routine to have travis checked the conventions with every build
- added support for IntelliJ-checkstyle-plugin (for running local checks), also updated contribution.md