Yahooquery

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- Fix bug related to symbols that have characters that need to be url
encoded (^)

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- Allow for user to use a string as a list of symbols to pass to Ticker class.
For example, previous version would require user to pass
`['fb', 'msft', 'goog']` to retrieve those three symbols. Now, the user
can pass `'fb msft goog'` or `'fb,msft,goog'`.
- Allow user to pass string, as well as list, to `get_endpoints` method. For
example, `['assetProfile', 'balanceSheetHistory']` is equivalent to
`'assetProfile balanceSheetHistory'`.

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- Fill NA values from history dataframe. Event data (dividends and splits)
will be filled with zeros. Other columns (high, low, open, close,
volume, adjclose) will be filled with prior day's data.
- Fill NA values from options dataframe. Missing values are replaced with zero

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- Entire library makes asynchronous requests (missing piece was the
option_chain method).

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- Missing required library requests-futures in setup.py file

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- Add asynchronous requests with the requests-futures library
- Add "events" to the history dataframe (dividends and splits)