Page copy

This pull request adds a "copy" method to pages.

By default, this will make an exact copy of the page and place new page immediately after the old page in the tree.

Basic usage: ```page.copy()```

Options:

__recursive__ - Setting this to True will copy the entire subtree of the page as well (default is False)

__to__ - This lets you specify a different parent page to copy the page into. The page would be added as the last child of this page

__update_attrs__ - This is a dictionary of attributes and values to change before saving the copy. Eg: ```page.copy(update_attrs={title: "New Title", slug: 'new-slug'})```

This is ready for review/merge. I'll create the admin UI in a separate PR