Add copy/publish functions

[meisekimiu]

• What kind of change does this PR introduce? This PR adds copy/publish functionality to the node-sdk. In addition to adding copy methods for both records and folders, there's also a function to fetch the Public Root, so publishing functionality (which is just copying to Public in the back end) can be properly implemented by scripts. I've also added a function to fetch the My Files folder so that I can test copying from "My Files" to "Public".

• What is the current behavior? (You can also link to an open issue here) You cannot copy, fetch the Public Root, or My Files directories.

• What is the new behavior (if this is a feature change)? Adds those things.

• Other information: I'm a bit unclear on the design of the architecture we want on this project... should this be how it's implemented? I kind of have to do an ugly thing in the copy-publish.js example file I made where I have to explicitly check whether an Item is a record or a folder before calling the exact method on it.

[meisekimiu]

I've finished up a wrapper method that lets the user call a copy method on a ChildItemVO without having to distinguish its type by hand. This also cleans up the example code.

[cecilia-donnelly]

@meisekimiu does this need a re-review at this point?

[meisekimiu]

Thanks for the review @cecilia-donnelly! Can you check the README changes I made real quick? (I think after this we should probably stop adding notes to the README but this change is a simple enough thing to document in the README).

[cecilia-donnelly]

Those README changes look great! I especially like that you noted that copying to the public root is equivalent to publishing.