add showConditional expressions and conditional disabling

Added more complex logic for hiding or showing any element of the form. Also conditional disabling, not only through static readonly property.
But there're two lines which were unexpectedly added to the pr,
+              if (part.key && part.schema && angular.isDefined(part.schema['default'])) {
-                sfSelect(part.key, scope.model, part['default']);		
+                sfSelect(part.key, scope.model, part.schema['default']);
 and I can't figure out why they are there.

Also, it's kinda hard to write/read your spec :(