Fixed generator for neo4j 3.x version and started updating the README

There seems to still be some problems when generating with the template:

/Users/andreasronge/.rvm/gems/ruby-2.1.1/gems/active_attr-0.8.3/lib/active_attr/typecasted_attributes.rb:76:in `_attribute_typecaster': Unable to cast to type NilClass (ActiveAttr::Typecasting::UnknownTypecasterError)