Extend range of cacheable objects

[phptek]

At present, the type of objects able to be cached is limited to SiteTree, and the cache is primed via a BuildTask or (optionally) on page-load. We need to get out from the confines of SiteTree caching and allow arbitrary lists of DataObjects to be cached.

Off the top of my head, it shouldn't be too hard to pass a class-name or string of comma-separated class-names and have the BuildTask use these, rather than the baked-in SiteTree at present.