Support for DateTimeOffset

As mentioned in https://github.com/praeclarum/sqlite-net/issues/217
Saving a DateTime is probably not what people want in most cases.

DateTime represents the same date and time agnostic to the timezone, such as if you wanted to describe "the start time of thanks giving dinner" across multiple timezone, or opening hours of shops. 

From the article at http://msdn.microsoft.com/en-us/library/bb384267.aspx:
"...
Note
These uses for DateTimeOffset values are much more common than those for DateTime values. As a result, DateTimeOffset should be considered the default date and time type for application development.
..."