Refactored POMs towards consistency

[dashorst]

This pull request fixes the compatibility between Select 2 and Wicket 7. In order to fix the dependencies I've also made the POMs consistent.

• All duplicate information was moved to the parent pom, the description of the parent POM is no longer the same as the Wicket CDI module.
• All dependency information is now centralized in the parent POM, including the plugins necessary for building.
• I've included the enforcer plugin for consistent POM layout (which is annoying, but helps in the end for enforcing a single style across all project modules).
• All plugins now use their most recent version.
• Dependencies now use the most recent Java 1.7 compatible version (json has a newer version, but requires java 1.8)
• Fixed all compilation issues with Wicket 7
• Examples still work.