Kraken: fix clang compilation problem

Fix #327 and close #328
Clang was not compiling the '+' DateTime operator (and after search he
was right not to, thanks @skywavenl:
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
Kraken: corrections of clang warning
Update readme.rst
Merge pull request #330 from antoine-de/fix_clang

Fix clang