Added support for Binance Smart Chain API url

[marciuz]

As far as my - potentially incomplete - information goes, bscscan.com works exactly like etherscan.io, being a fork of it.
To allow calls to bscscan.com you can then simply configure the URL, methods and actions should be the same.
Therefore it is possible to make calls with :

$client = new \Etherscan\Client($YOUR_APIKEY,  \Etherscan\APIConf::NET_BSC );
$res = $client->api('account')->balance($YOUR_WALLET_TOKEN);

[w3spi5]

I've created a similar package based on maslakoff's php-etherscan-api.
you can find here : https://github.com/w3spi5/php-bscscan-api.
Does this meet your PR ?