Async-v20

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- Added get_position_book and get_order_book API calls

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- series() method converts both UNIX and RFC3339 time's to pandas.Timestamp 's

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- Initialization doesn't freeze after failure
- Order methods exposes all arguments

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- Beta release. At present time client is considered feature full
with 100% test coverage
- _fields attribute stored on instance not class
- RESTful account() method added
- close_all_trades() method added
- Added replace() method to Model
- Simplified Endpoint decorator (No serial requests)
- Changes close_trades to close_trade (Method can only close one trade)
- Response parser checks HTTP status first
- Added tests

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- `type` argument is set automatically for subclass that define it
- implementation improvements

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- async_v20 objects are now immutable (greatly reducing complexity)
- Objects now have a repr
- removed inflection as a dependency
- Higher test coverage