Force use of fallback chanpool on App Engine

This is to work around a bug where App Engine, which as of this writing
runs Go 1.2, builds code which uses the "go1.3" build tag. See #67.

This bug is being tracked at:
https://code.google.com/p/googleappengine/issues/detail?id=11346

web/chanpool.go

@@ -1,4 +1,4 @@
-// +build !go1.3
+// +build !go1.3 appengine
 
 package web
 

web/cpool.go

@@ -1,4 +1,4 @@
-// +build go1.3
+// +build go1.3,!appengine
 
 package web