Folder enhancements

Folder functionality could use some improvements from current implementation.

Potential ideas:

• The /api/datasets/:id/files list includes all files (including those in folders and subfolders) but doesn't indicate which folder they are in (or if they are in a folder at all). https://github.com/clowder-framework/clowder/issues/34
• There is no way I see to get a list of files in a specific folder. https://github.com/clowder-framework/clowder/issues/35
• The 'Download All Files' button on the GUI is somewhat misleading inside a folder/subfolder:
  • 
  • Here, I am in a subfolder 'tiny little foods' and it would be reasonable for someone to expect the button to just download files I'm seeing in the folder. 'Download Dataset' would be a better label (if we don't want to support people downloading folders individually - which would have metadata implications for the idea of dataset vs folder).
• Dragging and dropping a folder from the operating system into the file upload GUI will correctly scan the folder and subfolders, but it adds all the files as a flat list and the folder organization is lost. I would expect the folder structure to be preserved. https://github.com/clowder-framework/clowder/issues/36
• On the UI, files have a 'Move' button to move into and out of folders, but the folders themselves do not have such a button. I don't know if the API supports adding a folder to another folder either (see below). https://github.com/clowder-framework/clowder/issues/37

Current folder API features for reference:

• Create a new folder
• Move file in a dataset into a folder
• Move file in a folder back out into a dataset (no folder)
• Delete a folder
• Update folder name
• Get list of folder names and IDs by dataset ID