Add native same-FS copy + refactor copy/move logic + add default copy/move to FileSystem

[ussuri]

@rpaquay: code @rginda: updated help in cp.js and mv.js

• Add a FileSystem.prototype.copy() stub, similar to move().
• Implement a native copy() for GDriveFileSystem.
• Move the copy/move logic from cp.js/mv.js to FileSystem (for same-FS) and FileSystemManager (for cross-FS, while delegating to FileSystem for same-FS).
• The above also means that any FileSystem now has at least a generic copy/move (e.g. DomFileSystem didn't have either before).

[rginda]

LGTM