Folders

[sanderpick]

Closes #68 (different implementation than originally expected)
Closes #69 (different implementation than originally expected)

This adds folder handling to the Files API, which involved removing the Files collection and the File model in place of Folders and a Folder model. Files are handled under the hood by the IPFS Files API. Folders are just IPFS Directories.

Demos

• Adding folders and files: https://asciinema.org/a/cBKjOToGtrUOFQbE5HZfPXaYi
• Inspecting folders and catting files: https://asciinema.org/a/1hvJGUDrgFLcNLJAiGRcrfRq1
• Removing files: https://asciinema.org/a/f1Ci5O6pDUuT7EhQkOlvRIwwA (notice folder root path changes)

Next Steps

This PR adds some low-ish-level APIs and commands for managing files and folder. Next, I'd like to add some higher level commands for syncing folders (these may require some merge handling, not sure yet):

• textile push <folder> (push latest)
• textile pull <folder> (pull whatever the remote has)

[sanderpick]

A project can only have one folder by a given name. Question: Maybe we can use "bucket" instead of "folder"? Too close to S3?

[jsign]

Very nice! 💯
I left some questions and comments.

[asutula]

Not much to add considering @jsign's thorough review. Looking forward to reading the replies on his comments. Looks good in general.

[sanderpick]

Thanks for the review, guys! I've addresses or created issues for the relevant comments.