Added NetBuffer.WriteAt()

git-svn-id: http://lidgren-network-gen3.googlecode.com/svn/trunk@347 e2304582-335f-4349-fc35-ba3f7475a1fb
ReadAllFields and ReadAllProperties now throws ArgumentNullException

git-svn-id: http://lidgren-network-gen3.googlecode.com/svn/trunk@348 e2304582-335f-4349-fc35-ba3f7475a1fb
adds .gitignore