Async-v20

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- Added guaranteedStopLossOrderRestriction to Instrument definition
- Added unixTime attribute to orderBook definition
- Added tags attribute to Instrument definition
- Added guaranteed to StopLossOrder definition
- Replaced ellipsis (...) with sentinel object
- Added fullVWAP to Transaction definition
- UnknownValue Exception is now replaced with UnknownKeywordArgument Warning
in order to make the client more robust to breaking changes from OANDA
- Checked many class definitions against updated OANDA documentation

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- OandaClient.close is now a coroutine

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- All OrderRequests now require an `instrument` as there first parameter.
This is to allow correct rounding of decimal numbers to the instruments specification.
- `guaranteed: bool` has been added to Transaction definition.
- Default SinceTransactionID value is now limited to the passed 900 transactions.
- `OandaClient.account()` method will use `get_account_details()` method when default
`SinceTransactionID` has expired.
- `OandaClient.get_pricing` now accepts an `InstrumentName` for `instruments`

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- `guaranteed: bool` has been added to Order definition.

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- Array class lazy instantiates objects
- Model objects with same attributes now compare equal (proving equal DateTime format)
- Improved repr of Array objects
- Improved implementation of `get_instrument`/s, `get_id`
- All Array objects have `get_instrument` and `get_instruments`. Returns either the first or all matching objects
- Removed Model class `_template` attribute as it was obsolete
- Added `get(self, name, default=None)` method to Model to allow for dict like key access.

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- Changed `get_transactions` method to `get_transaction` (API call can only return one transaction)
- Removed default parameter argument for `get_transaction`.(**transaction_id**) it is a required parameter.
- Made OandaClient.hosts a protected member. Now OandaClient._hosts
- `Array`. **get_instruments()** returns an Array of the same type this is when there is a one to many relationship
between the instrument and objects the array contains. `Array`. **get_instrument()** Returns the single object
when there is a one to one relationship. Both methods are mutually exclusive
- All objects now lazy instantiate attribute instances.
- Large refactoring of async_v20.definitions.base