Added query with comments and more model data.

- --comments flag is now available for queyring
- new mappings:
-- Account.username as String
-- Change.comments as List<Comment>
-- Comment.reviewer as Account
-- Comment.message as String
-- PatchSet.author as Account
-- PatchSet.approvals as List<Approval>
-- PatchSet.parents as List<String>
- replaced Approval.username (as String) by Approval.by (as Account)
Review feedback:

- Added tests for Account::username and Change::comments
- Avoid empty ArrayList when JSON is missing (or object not initialized)
Reverted wildcard imports
Added unit-test for Approval deserialization compatibility
Fixed Approval deserialization

Note that using 'transient' will prevent any XStream error, but it won't
 read the field at all, also preventing any migration effort.
Merge pull request #15 from gluck/newmodel

Added query with comments and more model data.