Removes null object from ClassEnum::Base.build

The null object was originally added to retain backwards compatibility
with existing Formtastic support. It was a pain to maintain support for
Formtastic, so support was removed, but the null object remained. The
null object breaks expected behavior if you're expecting the value to be
nil since it does not evaluate to a falsy value.

Previously, blank values had the same methods as enums:

  alarm = Alarm.new(priority: nil)
  alarm.priority.nil? # => true
  alarm.priority.low? # => false

Now, the value will be nil:

  alarm = Alarm.new(priority: nil)
  alarm.priority.nil? # => true
  alarm.priority.low? # NoMethodError: undefined method `low?' for nil:NilClass
  alarm.priority && alarm.priority.low? # => nil
Simplifies example for comparing enum values in select
No longer needs to check for anonymous classes

This was left over from the null object functionality that was
previously removed.
Removes serialize_as_json option from classy_enum_attr

This was originally added as a way to expose a JSON version of the enum
instance, but it doesn't really give you anything. If you want to
serialize the enum instances, you can simply override as_json in the
enum classes.
Removes support for Ruby < 2.0

Experimenting with keyword args and they don't work in 1.9.3. Not sure
if it's worth removing support for < 2.0, but that seems like a pretty
old version at this point.