[FLINK-1042] Changed ClassLoader field to be transient.

Serialization of the `RuntimeStatefulSerializerFactory` fails because the field `ClassLoader loader` is not serializable. The fix makes the field `transient`.

Author: Fabian Hueske <fhueske@apache.org>

Closes #92 from fhueske/StatefulSerializerFactoryFix and squashes the following commits:

c01cc27 [Fabian Hueske] [FLINK-1042] Changed ClassLoader field to be transient.