[Code Cleanup/Contexts] Added a context for 'attachment link', and moved both attachment contexts to proper Context_* classes.
* [Web-API/Attachments] When searching for attachment links using the Web-API, attachment filters can now be used: guid, display_name, mime_type, sha1_hash, size, storage_extension, storage_key, storage_profile_id, and updated.

* [Web-API/Attachments] When displaying attachment links using the Web-API, attachment placeholders can now be expanded.  Previously this just displayed the attachment link information and required additional requests to load full details.
* [Web-API/Usability] The Web-API will now properly removed nested _labels and _types from objects and search results when they were lazy-loaded.  Previously these were being repeated on every search result, despite the 'results_meta' key providing the same values.

[Code Cleanup/Web-API] The 'show_meta' option is now handled by the success() method, rather than inside handlePostSearch().
* [Web-API/Usability] In the Web-API, the 'show_meta' option now works when retrieving individual records as well.  This removes the '_labels' and '_types' dictionaries that describe the keys, which saves bandwidth if this information is already known (e.g. cached in the client).
* [Web-API/Usability] Fixed an issue in the Web-API where lazy-loaded 'message' objects always showed 'ticket' placeholders even if the messages were already contained inside a ticket.  This didn't cause any problems, it just added a bunch of redundant placeholders (e.g. ticket->message->ticket).
6.7.2-checkpoint