Add new SHOGun2 module: shogun2-util

The new module currently only provides a customized JSON/Jackson Object Mapper for the needs of SHOGun2 (e.g. for the serialization of Joda date types).
Remove unneeded dependencies from shogun2-web
Let the archetype use the customized ObjectMapper
Merge pull request #39 from buehner/json-date-serialization

Add util module and serialize joda dates in ISO8601