All dates should be independent of locale settings

[johnerikhalse]

Utility code creates new instances of java.text.SimpleDateFormat without explicitly setting the locale. OpenWayback does not seem to be influenced by this, but creation of (w)arcs might be.

[johnerikhalse]

I've made a pull-request which fixes this (#22). Be aware that if OpenWayback is updated to use this fixed version, one unit test will break on machines with non-english locale, because the test itself is not locale-safe.