title: ext.data.record conditional validation
body: <p>I am working w/ Ext 3.3
  How can I do conditional value checking, on the Ext.data package level, like; 
  if the person is married, require spouse name, otherwise let it be blank.</p>
  
  <p>The solution I am looking for should be on the data package. (lets say that I dont have much control about how data is edited, but I am responsible for validating it.)
  Ext.Data.Field lets me say allowBlank, true or false. I wonder if I can pass a function there which accepts the record, and returns true or false based on the other fields on the record.</p>
  
  <p>I am looking for an alternative solutions for this, on the Ext.data package, (on the Store or Record level).</p>