[BUGFIX #484] Improve NestedTransaction handling

* ensure running `commit` always clears the current transaction
* if we begin a transaction with an existing transaction warn, commit it, then start begin the next.

packages/@glimmer/runtime/lib/environment.ts

@@ -330,7 +330,8 @@ export abstract class Environment {
   }
 
   begin() {
-    assert(!this._transaction, 'Cannot start a nested transaction');
+    let transaction = this._transaction;
+    assert(!transaction, 'a glimmer transaction was begun, but one already exists. You may have a nested transaction');
     this._transaction = new Transaction();
   }
 
@@ -359,8 +360,9 @@ export abstract class Environment {
   }
 
   commit() {
-    this.transaction.commit();
+    let transaction = this.transaction;
     this._transaction = null;
+    transaction.commit();
   }
 
   attributeFor(element: Simple.Element, attr: string, isTrusting: boolean, namespace?: string): AttributeManager {