Organization of Image/Document assets

[HeyJoel]

It should be easier to organize and manage assets in large asset collections. Typically you might see this in other CMS's as a folder/file structure, but we also have a tagging system that could be better visualized when browsing.

Certainly better search and filtering UI would also make a difference here. Free-text search is detailed in #31.

Batch operations might be needed, batch upload is covered by #7 but please comment on any other features you feel are necessary.