Prevent DoS through XML entity expansion

Add a ProtectedXMLParser that overrides the
doctype declaration handler. The handler simply
throws an exception and prevents any further
parsing of the incoming xml.

Fixes LP Bug #1100282

Change-Id: I6488e1a6a52326006e7e7927ece5b5939b72e83e
Merge "Prevent DoS through XML entity expansion"