Add addClassLoader method to JavassistTemplateBuilder

The commit 364c4737699411ab6f530e7a7a485a81ada17a13 removed
addClassLoader method.
The method addClassLoader should come back again, because
this method seems to be still useful and is used in some
applications(e.g. msgpack-scala).

Signed-off-by: Tsuyoshi Ozawa <ozawa.tsuyoshi@gmail.com>