Makes some attributes optional in API response to get faster/lightweight responses.

These attributes are not required for common uses case (eg. updating an object). They can be requested in the reponse using the 'include' parameter. Example GET /issues/1.xml?include=journals. The list of attributes that can be included in responses will be documented in the wiki.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@4486 e93f8b46-1217-0410-a6f0-8f06a7374b81