[rules-dev] Fix to GUVNOR-358

[Jervisliu]

Hi,

I am working on https://jira.jboss.org/jira/browse/GUVNOR-358, please 
see my comments for details. There are two ways to fix this problem. One 
is making org.drools.runtime.rule.AccumulateFunction extends from 
Externalizable interface. Another solution is making it clear both in 
doc and samples that custom AccumulateFunction needs to implement both 
AccumulateFunction and Externalizable interfaces. As this is more like a 
usability issue, I would like to ask your opinions.  Which way do you 
prefer? Thanks.

Cheers,
Jervis