Added property types

composer.json

src/CryptoCurrencyChart/API/Struct/BaseCurrencies.php

src/CryptoCurrencyChart/API/Struct/Coin.php

src/CryptoCurrencyChart/API/Struct/CoinHistory.php

src/CryptoCurrencyChart/API/Struct/Coins.php

src/CryptoCurrencyChart/API/Struct/DataTypes.php

src/CryptoCurrencyChart/API/Struct/HistoryData.php

src/CryptoCurrencyChart/API/Struct/ViewCoin.php