Added a trait exemplar for Typed Criteria DSL.

Since filters (criteria) are concerned with constraining a result
set, they do not necessarily have to exist as a type which will
be instantiated.  This change serves to verify that fact.

samples/src/test/scala/dsl/criteria/TypedCriteriaSpec.scala

@@ -23,10 +23,15 @@ import org.scalatest.junit.JUnitRunner
 import reactivemongo.bson._
 import reactivemongo.extensions.dsl.criteria._
 
+trait Company {
+  val name: String;
+  val employees: Int;
+}
+
 case class Person(val firstName: String, val lastName: String, age: Int)
 
 /**
- * The '''TypedCriteriaSpec''' type verifies the
+ * The '''TypedCriteriaSpec''' type verifies the expected behaviour of the
  * [[reactivemongo.extensions.dsl.criteria.Typed]] Criteria DSL.
  *
  * @author svickers
@@ -64,5 +69,28 @@ class TypedCriteriaSpec
         )
       );
     }
+
+  it should "support trait-based expressions" in
+    {
+      val query = criteria[Company].employees === 42 ||
+        criteria[Company].employees < 10;
+
+      BSONDocument.pretty(query) shouldBe (
+        BSONDocument.pretty(
+          BSONDocument(
+            "$or" ->
+              BSONArray(
+                BSONDocument(
+                  "employees" -> BSONInteger(42)
+                ),
+                BSONDocument(
+                  "employees" ->
+                    BSONDocument("$lt" -> 10)
+                )
+              )
+          )
+        )
+      );
+    }
 }