Nesting Sites

[rmhasan]

This is a PR to help solve issue dropbox/nsot#215. Added -p option to nsot sites add and
nsot sites update. It allows users to set the parent in nsot's Site model to the site id of the parent site. This PR is a part of dropbox/nsot#254.

[rmhasan]

The .travis.yml file is out of date. In order to test nested sites, you need to first merge dropbox/nsot#254. Then update .travis.yml to pull the most recent version of nsot. Then the tests for list, add and update should pass.

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