Add KeyStats Resource

Fetches company's key stats for a symbol

Example:

key_stats = IEX::Resources::KeyStats.get('MSFT')

key_stats.symbol # MSFT
key_stats.market_cap # 825814890000
key_stats.market_cap_dollars # '$825,814,890,000'
# and more...

For more information, check: https://github.com/dblock/iex-ruby-client#get-key-stats