Create DefaultSerializer so that as_json uses same interface.

This is to ensure that PORO's as_json is called if no serializer
is specified.

Original behaviour was that serializable_hash was being called,
overriding the as_json method.
Move DefaultSerializer from array_serializer.rb to serializer.rb
Remove pry-nav from gemspec.
Restore order of fields in hash to original order.
Merge branch 'create-default-serializer' into HEAD

Conflicts:
	lib/active_model/array_serializer.rb
Remove line break on the conditional statement.
Merge pull request #207 from stantona/create-default-serializer

Create DefaultSerializer so that as_json uses same interface.