Bug fixes for codecs.

Added proper serialization of Empty Type Definitions into TransformerGenerator.xtend.
Added overload method "toDocument" which creates XML Document without schema definition for particular Data DOM tree.
This fix is primary needed for md-sal NetconfMapping. If this fix is NOT applied the toRpcMessage in NetconfMapping.xtend will NOT work properly.
Added javadocs for both toDocument methods.

Signed-off-by: Lukas Sedlak <lsedlak@cisco.com>
Change-Id: Ia1af32d3a9f11d81da8abf4274b84ca3b5d610d5