Improved pagination in GCE

This pull request improves the pagination in GCE so it no longer sucks :D The main changes are:

* Removed all `listFirstPage` and similar methods from all APIs.
* All *list* methods have two forms: one to list all records that returns a `PagedIterable` (no params), and one to get a concrete page that accepts a `ListOptions` object.
* The `ListPage` object representing a single page has been refactored and improved and now has a convenience method `toPagedIterable`, that allows converting it to iterate over the pages after a filter operation or similar.
* Refactored all parsers to accommodate the new pagination logic.

In summary, every API only has the minimum and meaningful methods to get a list of records, and the pagination domain objects have been reworked to make it trivial to iterate over the pages.

I've also removed most of the warnings in the project.

/cc @ccustine @danbroudy Could you give a try to the branch and run the live tests? Your feedback on how the new pagination methods look like from a user point of view is appreciated too :)
/cc @adriancole Your feedback is also welcome :)

I'd prefer `FluentIterable.from(api...concat());` or  `FluentIterable.from(api...flatten());` I think we should stop proliferating FluentIterable as it can easily wrap things and reduce the surface area we have wrt guava compatibility. (again. not interested in an angry debate with people, just offering advice).