Support composite conditions

[cduplichien]

Thinking like:

<conditional>
   <!-- Allow one of the following condition types -->
   <if-any>
      <!-- rules here -->
   </if-any>
   <if-all>
      <!-- rules here -->
   </if-all>
   <if-none>
      <!-- rules here -->
   </if-none>
   <if-some atLeast="x" atMost="y">
      <!-- rules here -->
   </if-some>
   <!-- Allow one each of following consequences -->
   <then>
      <!-- rules here -->
   </then>
   <else>
      <!-- rules here, could also be conditionals if you wanted to have "else if" behavior -->
   <else>
</conditional>

Rather than code additonal thing to support on top of our nested rules conditional structure, I'd rig it up so that the validation parser creates Conditional objects (or whatever the implementation is) when it reads nested rules. So a nested rule would essentially parse as a conditional if-any with only one condition rule and the children as the "then" consequences.