Added test to raise ImportError for missing yaml.

This test makes sure an import error for YAML is raised when it is not
installed instead of saying the serializer does not exist.
Added yaml import that raises ImportError.

This will return a meaningful exception to the user when the YAML module
does not exist and the requested serializer format is yaml.
Added test for dumpdata command + yaml.

Make sure dumpdata command raises an ImportError when the yaml module is
not available.
Added call to get_serializer.

Prior to the serialization optimization test, simply get the requested
serializer to make sure it exists.  This will allow a yaml format import
to raise an ImportError when the module is not installed.
Added test for yaml deserializer.
Refactored yaml test into its own function.

The yaml test logic can now be used for getting a yaml serializer and
deserializer.