Added tests for missing pyyaml.

This test makes sure an YAML import errors are communicated to the
caller rather than stating the serializer does not exist.
Added yaml directly into BUILTIN_SERIALIZERS.

The serializer definitely exists, but the dependent yaml module may not
be installed.  The register_serializer() function will catch exceptions
and will stub in a fake serializer object that will raise the exception
when the serializer is used.
Moved get_serializer() call in dumpdata command.

Moved the get_serializer() call within the condition that checks public
serializers.  This will allow exceptions other than
SerializerDoesNotExist to be raised in order to provide the caller with
useful information, e.g when pyyaml is not installed.
Updated NoYamlSerializerTestCase to run with yaml.

In order to verify the behavior of using the yaml serializer when yaml
is on the system, fake the ImportError when the serializer is being
registered.
Added name to AUTHORS (per Russell Keith-Magee)
Fixed existing tests to handle BadSerializer.

When a BadSerializer instance is stubbed in for the yaml serializer,
make sure tests do not fail.
Cleanup commit after peer review.
Merge pull request #1582 from rca/12756-missing-yaml-module-serializer-error-message

Fixed #12756: Improved error message when yaml module is missing.