Option to force DateTimes to deserialize as Utc

Things got a little messy in the last version of this PR (#20), so it seemed easier to just rebase from upstream/master and try again.

Here's the notes copied from there:

>The default DateTime deserialization will set the DateTimeKind to Local if there's timezone information in the string. This makes sense, but timezones are a >PITA to manage, and being able to guarantee everything comes out as UTC is pretty important (to me, at least).
>
>Adds JsonMapperOptions flags enum, and a static Options member in the JsonMapper class.
>
>The behavior is enabled by a global flag which is a little gross, so maybe some sort of JsonMapper context could be useful in the future.