Adding a nbviewer search to website

This change is to add search functionality to nbviewer so that we can search for notebooks based on topics. This takes advantage of google search functionality to look through nbviewer pages which are cached by google. 

To test this feature, you will need a google developer account with custom search enabled. Then you will need to get the api key and cx search code for nbviewer.ipython.org. Only the first 100 calls per day is free, but after that it will be $5 per 1000 requests up to 10,000 requests. https://developers.google.com/custom-search/json-api/v1/overview#pricing