BUG-1275: add TXW2 to dependency management

This small package contains IndentingXMLStreamWriter, which is useful
for pretty-printing XMLs which are output via XMLStreamWriter.

Companion change, Ic83e55eb6d0eeea599e909cc0dae3b145ef9cc9f, introduces
the wrapped user. The dependency may be removed at a later point if
someone creates an ODL class with equivalent functionality.

Change-Id: I834a73f6466c2829194184ee0a30bac6d093665d
Signed-off-by: Robert Varga <rovarga@cisco.com>