Add coverage of DiskType Api

I figured before adding support for SSDs it makes sense to add coverage of the DiskTypes Api. 

DiskTypes Api Documentation: https://cloud.google.com/compute/docs/reference/latest/diskTypes 

The implementation is very similar to MachineTypes Api. Refactoring redundancies may be desirable in the future. 

Also like MachineTypes this only covers the "get" and "list" operations and does not yet cover the "aggregatedList" operation.  

 
jclouds APIs with pagination usually provide two methods to implement the *list* calls:

* One method that does not have pagination parameters and returns a `PagedIterable` (an Iterable that knows how to jump to the next page).
* And a method that accepts custom options (such as the page number, marker, whatever), that returns an `IterableWithMarker` (a single page, with the marker to the next one, that can be turn into a `PagedIterable`, if needed).

With this approach, would the last two *list* methods in this class be enough? The previous `listFirstPageInZone` and `listAtMarkerInZone` seem redundant?
Also, could the last method (the one that accepts the `ListOptions` parameter) be changed to return just an `IterableWithMarker`?