Update maven configuration

Fix for https://github.com/protostuff/protostuff/issues/41

* move common plugin configuration to <pluginManagement> section
* set version for all plugins
* replace deprecated `pom.version` with `project.version`
* replace deprecated `pom.url` with `project.url`
* add maven-enforcer-plugin, enable "dependency convergence" rule: http://maven.apache.org/enforcer/enforcer-rules/dependencyConvergence.html (ensure that there are no inherited artifacts with different versions)
* add missing dependencies to <dependencyManagement> section