BUG/TST: Yahoo Finance (a foundational resolve of GH: #2847,  #2853)

Resolve issues in `pandas.io.data.py` and `test_yahoo.py`.

- BUG: Fixed decode issue in `get_data_yahoo()` for python 3.3 (resolves #2847)

- EHN: Updated `_calc_return_index()` function to properly handle groups of multiple stocks that do not share the first valid price on the same date (i.e., stocks whose IPO's dates exist within the query date range). Ensures that all stock's first values in `ret_index` are set to 1, regardless of actual initial starting date.

- TST: Modify test suite for yahoo finance functions in `test_yahoo.py` to accurately test queries to yahoo finance (resolves #2853/#2847)