Added some functionality to return `pandas.Series`

The currently functionality only allowed to write the list of s&p 500 tickers and russell 3000 tickers to a `.csv.`  I did two things:

1. extracted the general functionality you build into "helper_functions", called:
  a. sp_helper_fun()
  b. russell3000_helper_fun()

2. I kept the name of your functions (that were used by other parts of the module **the same**, so that this merge request would be backwards compatible (i.e. all of your functions were still work by invoking the same code).

3. Then I added `fun-name`+ _tickers_to_series() to allow the user to return a `pandas.Series` of either the Russell 3000 or S&P 500, again using the helper function that chunks out all of your beautiful functionality.

Great job on this module!!

-B