Some rules should be recategorized as code smell instead of bugs

[hosszubalazs]

Hi!
Some issues like :
grvy:org.codenarc.rule.imports.UnusedImportRule
grvy:org.codenarc.rule.unused.UnusedPrivateFieldR
should be set to Code Smell instead of Bug.

In general the Groovy rules should follow the Java rule categories.

Thanks and cheers,
Balázs