EHN: Improved Yahoo finance api functionality

Added functionality of listing stock index components via query to yahoo finance
API. Expanded existing functionality of other yahoo finance features in 
`pandas.io.data.py`, in detail:

- Created new function `get_component_yahoo()`, allowing for query of stock index
  components. Returns DataFrame containing list of component information for 
  index represented by `idx_sym` from yahoo. Result includes component symbol 
  (ticker), exchange, and full name.

- Refactored `get_data_yahoo`, now allows for multiple stock symbols with input
  of list-like object (tuple, list, Series), or DataFrame with stock symbols as index.
  Includes convenience options for adjusting price and adding return index.

- Refactored `get_quote_yahoo`. Can now accept a single symbol (str) along with 
  list-like object (list, tuple, Series).

- Expanded test coverage to the new features.