Add ionomy exchange

[MitchellCash]

• What is the purpose of this Pull Request?
  Add ionomy exchange.

• I have added Specs

• (If implementing Market Ticker) I have verified that the volume refers to BASE

• (If implementing Market Ticker) I have verified that the base and target is assigned correctly

• I have implemented the trade_page_url method that links to the exchange page with the base and target passed in. If not available, enter the root domain of the exchange website.

• I have verified at least ONE ticker volume matches volume shown on the trading page (use script below)

client = Cryptoexchange::Client.new
pairs = client.pairs 'exchange_name'
tickers = pairs.map do |p| client.ticker p end
sorted_tickers = tickers.sort_by do |t| t.volume end.reverse

[MitchellCash]

Apologies, noticed a small bug in the ionomy API. Will fix and update, please hold off merging until then.

[MitchellCash]

PR updated to reflect latest ionomy API. Ready for review/merge.