Add option for SPEL rule execution

[twicksell]

Would like to have namespace support for using SPEL as the rule implementation instead of a concrete RuleExecutor.

[cduplichien]

Since the rule args already accept EL conditionals, you could very quickly make a rule executor implementation that just returns the provided argument. Not as pretty... but recommend this workaround for 0.6.2 and save the namespace updates for the 0.7.0 overhaul.
It'd be like <rule:conditional arg="${some boolean SPEL expression}"/>

[twicksell]

That doesn't seem like it will work for a lot of cases that I'm dealing with. Say I have some class foo which has a property bar. Bar has validation rules of its own, one of which I want to use SPEL for. Now, when I validate foo, and get to the rule on bar, the argument will be evaluated with foo as the root context. Something about that implementation would need to be tweaked in order to make this work. I really want to evaluate expressions on path being validated, not against whatever the root object might happen to be.

[cduplichien]

Can figure this out following rule consolidation