Yahooquery

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- Made the symbols argument to the `Ticker` class a required argument
- Fixed bug related to the `fund_category_holdings` property.
- Fixed bug related to the `history` method.
- Added tests and initial attempt at Travis CI

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- Added frequency arguments to `balance_sheet`, `cash_flow`, and
`income_statement` methods. They will default to annual, but can
return quarterly statements with "q" or "Q" arguments.
- Added a `calendar_events` property to the `Ticker` class.
Shows next earnings date, previous dividend date, and other metrics.

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- Fixed bug related to formatting empty lists

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- Add `fund_performance` property to the `Ticker` class. Shows
historical fund performance as well as category performance.