Transactions

* dsl, state checks
* define transaction sql commands in connection

spec/dummy_driver.cr

@@ -42,11 +42,34 @@ class DummyDriver < DB::Driver
       @connected = false
     end
 
+    def create_transaction
+      DummyTransaction.new(self)
+    end
+
     protected def do_close
       super
     end
   end
 
+  class DummyTransaction < DB::TopLevelTransaction
+    getter committed = false
+    getter rolledback = false
+
+    def initialize(connection)
+      super(connection)
+    end
+
+    def commit
+      super
+      @committed = true
+    end
+
+    def rollback
+      super
+      @rolledback = true
+    end
+  end
+
   class DummyStatement < DB::Statement
     property params