Allowing sites to be nested in other sites

[rmhasan]

This is a PR for #215. This PR allows Sites to be nested in other sites. I performed the following tests. I also created a pull request for pynsot that allows users to pass the id of the parent site. The command line argument for passing the parent id is -p

EDIT: I made the pull request dropbox/pynsot#124

$ nsot sites add -n first
# site with name 'first' is added

$ nsot sites add -n second -p 1
# site with name 'second' is added and its parent is 1

$ nsot sites remove -i 1
# site was not removed, because it is second sites parent

$ nsot sites update -i 1 -p 1
# fails because a site cannot be nested in itself

$ nsot sites add -n third -p 2
$ nsot sites add -n fourth -p 3
$ nsot sites add -n fifth -p 4
$ nsot sites update -i 1 -p 5
# update fails because site with id 5 is a child or grand child of site with id 1

[jathanism]

@rmhasan Thanks for this contribution, too! Me and the other maintainers will be taking a look and try to keep this conversation going.

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