Support validating extra action properties.

Setting package variable $Workflow::Factory::VALIDATE_ACTION_CONFIG
to a true value causes $action_class::validate_config() to be called
if it exists.  This is useful if you have some tunable parameters
for your actions and you do not want to wait until run-time to
validate them.
Do not call validate_config() if it does not exist.

In addition, fixed a bug in the way function existence is checked.
test: observer sub existence not checked correctly
Fixed bug: correctly check observer sub.
Merge pull request #2 from dtikhonov/master

Pulling request...