Parsing time values from Wikibase to datetime object

[dpriskorn]

The time value is "+2007-03-31T00:00:00Z"
I'm thinking it is a common use case to want this parsed into a datetime object and maybe we want WBI to support that?

[LeMyst]

Added in #542

datetime can't be used with year < 1 :(

And i'm reluctant to add another dependency in WBI