Change type of date/time fields from 'string' to 'DateTimeOffset'

Hi, this is my first PR to your repo, let me know if there are things you'd like done differently.

The Zendesk API v2 requires that most date/times be formatted as ISO8601. In the ZendeskApi_v2 library, the types of date/time fields is always `string`.

It would be helpful for it to be `DateTime` or `DateTimeOffset` so that developers don't pass incorrect date/time formats to the Zendesk API. For instance, there have been Zendesk API developers who have passed strings like `Friday, September 9, 2014 8:03 AM` for `due_at` to the Zendesk API.

It also is helpful for developers when receiving values back from the Zendesk API; if they're already of type `DateTime` or `DateTimeOffset` in the returned result, the developer doesn't have to worry about parsing the result.

I chose `DateTimeOffset` over `DateTime` since it includes timezone information and is the more modern choice in the .NET Framework. (Does this make sense?)

This is probably a breaking change so I suppose it'll have to be carefully considered. However, I think most developers would prefer to work with a real date/time object instead of a string in this case.

 * Changed date/time types from `string` to `DateTimeOffset?`
 * Added unit test for creating a ticket with `DueAt` setting
 * Add check for `CreatedAt`, `UpdatedAt` to ticket creation test