Add jclouds-dto project

This pull request add jclouds-dto to labs.

This project intends to provide "data transfer objects" for most of the jclouds domain objects.
The idea is to have objects that are easily serialized so that they can be used for exposing jcouds to jmx, etc.

The project consists of 2 modules:
i) dto-core: Which provides the dtos
ii) dto-converters which provides functions for converting the domain objects to dtos.

Note: In some cases I am using an ImutableSet builder, over guavas lazy iterables (e.g. transfomed iterables), mostly because the lazy stuff had some issues with serialization. I am not aware of any better way (an other idea is to create a set manually?).