Get TF2Items building on 64-bit

- Pull alliedmodders/sourcemod#2107 wholesale to modernize the
build script (multiplat, hl2sdk-manifests, safetyhook) and make
some minor adjustments to support CDetour.
- Stub out plugin compilation for now; it will be added in a
separate commit to preserve readability.
- Hoist reading 'tf2.items.nosoop' and add it as a fallback for
GiveNamedItem.  SM's gamedata updater will stomp all over any
changes we make in asherkin's.
- Fix sizing for reverse-engineered TF2 classes to match the 64-bit
release.
- Rename smsdk_ext.hpp since CDetour is picky about the filename.

.gitignore

@@ -8,3 +8,6 @@
 /extension/msvc10/tf2items.sdf
 /extension/msvc10/tf2items.sln
 /extension/msvc10/tf2items.suo
+
+hl2sdk-manifests/
+safetyhook/