Provide ccxt connection market data fallback

There are cases (though rare) where exchanged delist or temporarily remove access to an asset. In this case, the API of the exchange may no longer provide any market data about said asset.

The specific use case that prompted this was BSV's temporary unavailability on Bittrex due to a recent 51% attack on the BSV chain. As a result a Kucoin was implemented as a fallback for Bittrex. Currently Bittrex is the only ccxt connection that has a default fallback, others may be implemented in the future.

Changes to stablecoin default pricing & fee pricing

• Nexo connection no longer defaults stablecoin prices to $1 in order to balance asset quantities and values.

Change transaction "feePrice" key to "feeUsdPrice"

• provides an easier and more unified transaction API going forward
• fixed previous fee pricing inconsistencies

Hot Fix: NaN baseUsdPrice on Coinbase Buy/Sell Transactions

This issue was caused by not providing a baseQuantity fallback value for ccxt connections. This problem only manifests in Coinbase as Coinbase's buy and sell transactions use ccxt's "amount" value rather than the "filled" value.

Some other linting and minor fixes were packed in too.

Implement Swap Transactions

Swap transactions are used to represent a transaction in which the base and quote currencies are crypto (A.K.A. no fiat!).

Swap transactions are intended to provide a clearer line between how fiat and cryptos interact through transactions. This allows us to reduce the scope of buy and sell transactions to only deal with fiat quote currencies and crypto base currencies.

Implemented on the following connections:

• Bittrex
• Coinbase Pro
• Kucoin
• Nexo
• Etherscan

Finalize CI/CD Pipeline

Finalize integration with Travis CI and knapsack

Bug Fixes

• fixed multiplication error when finding USD total value for Etherscan orders
• export Transaction and Order types from module

Initial Stable Version

Implements connections to

• Bittrex (API)
• Celsius (API)
• Coinbase (API)
• Coinbase Pro (API)
• Ethereum Address (Etherscan API)
• Kucoin (API)
• Nexo (CSV)