Async-v20

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- Changed incorrect naming _i_ds to _ids
- Fixed OrderRequest formatting when a OrderRequest instance is passed that defines an instrument.

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- Fixed InvalidFormatRequest Error when OrderRequest does not define an `instrument` attribute

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- Fixed incorrect handling of stream response when endpoint returned status 400

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- Added health API and initialization check
- Fixed stream_transactions. Now returns correct type
- Changed streaming responses keys. Where stream_pricing() returned 'PRICE' or 'HEARTBEAT'
response now contains 'price' or 'heartbeat'. Like wise stream_transactions() now returns
'transaction' or 'heartbeat'. This has been done to standardise access to the transaction
stream response and 'heartbeat' objects. Due to the different types these objects may take.
- Added async_v20.exceptions module
- Added logging to modules

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- Added default argument to Array.get_id & Array.get_instrument
- Removed default value from get_candles(*count*)

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- Fixed bug in rest implementation. Positions now update correctly