Add a clone method to Tag and NavigableString

[Martijn Pieters]

BeautifulSoup does not support cloning an element (with all contained child page elements); use cases include creating multiple copies of a sub-tree to generate a larger document quickly.

You cannot use copy.deepcopy() for this task as elements maintain relationships to their tree parent and siblings; a deepcopy would needlessly copy those elements along. NavigableString also implements an incorrect __copy__ method that returns `self` without taking the mutable parent and sibling references making this approach unworkable.

I've attached a patch that adds explicit cloning support; new `Tag.clone()` and `NavigableString.clone()` methods which create copies that are *not attached to the document*. With it you can repeatedly create copies of subtrees without affecting the original tree.

This patch assumes that the patch in bug #1307471 has been applied; in my testing (with lxml as the parser), a freshly created BeautifulSoup tree does not have `.builder` set on the created elements.

This code was initially written up as an answer to a question on Stack Overflow, see http://stackoverflow.com/a/23058678/100297

[Leonard Richardson (leonardr)]

I've adapted this patch to change the behavior of __copy__.

[Leonard Richardson (leonardr)]

Revision number: 381.