Maven dependency management improvements

* 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
Maven plugin configuration improvements

* 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`
Merge branch 'master' into fix/dependency-management

Conflicts:
	pom.xml
	protostuff-maven-plugin/pom.xml
	protostuff-me/pom.xml
Fix build errors after merge with master (missing version for plexus in maven configuration)
Merge pull request #42 from kshchepanovskyi/fix/dependency-management

Update maven configuration