File transfer support

[barneygale]

Add methods like PathBase.copy() and copytree() that support copying files and directories from one path object to another.

PathBase.copy() should:

• Call open() methods on the source and destination to transfer data
• Attempt to copy permissions with stat() and chmod(), but suppress UnsupportedOperation
• Copy symlinks with is_symlink(), readlink() and symlink_to() if the user sets follow_symlinks=False (and if they fail with UnsupportedOperation, copy the symlink contents?)

PathBase.copytree() should:

• Call walk() to iterate over the source
• Call mkdir() to create directories, but suppress UnsupportedOperation
• Call copy() to copy files

Implementations in Path should use shutil when a os.PathLike argument is given, and call super() otherwise.