Use a string for class property keys

ActiveFedora::Base.class.properties are strings, so getting and setting
them should use strings for the keys. When setting values,
ActiveTriples::Resource won’t save the value, so we call it as a method
on the base object directly.
Raise error unless orm.new? fixes #409

Raise an error if we’re initializing a new ActiveFedora::Base object
using an orm resource that isn’t new. In other words, we don’t want to
create new objects with URIs that already exist in the repository.
Merge pull request #539 from projecthydra/existing-pid

Raise error unless orm.new? fixes #409
Merge pull request #538 from projecthydra/attributes

Use a string for class property keys