Merge pull request #29 from mdcb-gmt/master

handle Subscription using boost::shared_ptr rather than std::shared_ptr
Merge pull request #28 from oroulet/master

expose namespace methods, add tests
fix timestamp unit test
ToDateTime optional usec
Merge pull request #30 from mdcb-gmt/master

fix timestamp unit test
Merge branch 'master' of https://github.com/FreeOpcUa/opcua-python