[fix] JSON output of name contains invalid field

the current json output of a Name instance contains the field 'empty'
that is invalid for this SCIM attribute. this is caused by the
'isEmpty()' method getting serialized by jackson. fixed it by annotating
the method with @JsonIgnore.
Merge pull request #108 from tkrille/fixing-empty-in-json

[fix] JSON output of name contains invalid field