add opentxs namespace

Files left in the unnamed namespace:

include/otapi/ot_otapi_ot.hpp
include/otlib/OTAssert.cpp
include/otlib/OTCommon.hpp
include/otlib/OTWindowsRegistryTools.hpp
include/otlib/stdafx.hpp
src/otapi/ot_otapi_ot.cpp
src/otlib/OTAssert.cpp
src/otlib/OTWindowsRegistryTools.cpp