This Fixes #935 : Showing list of users who starred the current notebook

@gordonwoodhull :

The list is shown in a dropdown like this : 

![starrerlist](https://cloud.githubusercontent.com/assets/1859165/5106218/e3351e98-701b-11e4-9eb4-82b9f56aa7fd.png)

Also, as I mentioned in the commit description, this list is going to be generated dynamically for future notebooks. But for the old ones, we are collating the users (who starred) in a list each time the notebook is opened. This is expected to slow down the dropdown show the first time, specially if the number of users is large. 

Also, the snippet of code which does that will go obsolete after a time and it will never run. We can always use an alternative way, which is to run a script, prepare the data for all the notebooks and remove the script. As of now we have implemented it as a middle way out between running a migration script and generating the list every time for all notebooks.

Your thoughts please.

we use an underscore suffix to mean a member of a "class" (usually a top-level closure), and not for local variables