Fix clang

Fix #327 and close #328
Clang was not compiling the '+' DateTime operator (and after search he was right not to, thanks @skywave:
http://stackoverflow.com/questions/18596412/strange-error-with-a-templated-operator-overload)

he was attempting to instanciate the template with too much stuff and sometime had 2 pod which is not valid.

Restrict to any time_duration instanciation to be able to work with
boost::time_duration or navitia::time_duration