Merge pull request #1 from szpak/PojoBuilderFeature

#341 - minor tweaks in PojoBuilder
PojoBuilder changes following code review - added missing license, JavaDocs and converted tabs to spaces
#341 - copy FieldNode instances for new class

FieldNodes shouldn't be shared by different classes:
http://grails.io/post/15965611310/lessons-learnt-developing-groovy-ast-transformations