Added parsing of anchors, aliases, and tags (still no semantics yet).

Fixed a silly bug in the simple key pushing (queues are FIFO!).

test.yaml

@@ -1,10 +1,29 @@
----
-here's a sequence:
-  - item 1
-  - item 2
-now an inline sequence: [1, 2, 3]
-and here's a map:
-  name: Jesse
-  age: 23
-and here's an inline map: {state: Illinois, city: Urbana-Champaign}
-...
+--- !<tag:clarkevans.com,2002:invoice>
+invoice: 34843
+date   : 2001-01-23
+bill-to: &id001
+    given  : Chris
+    family : Dumars
+    address:
+        lines: |
+            458 Walkman Dr.
+            Suite #292
+        city    : Royal Oak
+        state   : MI
+        postal  : 48046
+ship-to: *id001
+product:
+    - sku         : BL394D
+      quantity    : 4
+      description : Basketball
+      price       : 450.00
+    - sku         : BL4438H
+      quantity    : 1
+      description : Super Hoop
+      price       : 2392.00
+tax  : 251.42
+total: 4443.52
+comments:
+    Late afternoon is best.
+    Backup contact is Nancy
+    Billsmer @ 338-4338.