Set maven plugin versions as properties

Defined all used maven plugin versions as properties in the root pom.
All modules reference the same versions.
Updated plugin versions:
maven-surefire-plugin: 2.11 -> 2.16
git-commit-id-plugin: 1.9 -> 2.15
maven-eclipse-plugin: 2.5.1 -> 2.9
maven-idea-plugin: 2.1 -> 2.2.1
doxia-module-docbook-simple: 1.2 -> 1.4
maven-surefire-plugin: 2.15 -> 2.16
maven-javadoc-plugin: 2.9 -> 2.9.1
cobertura-maven-plugin: 2.5.2 -> 2.6