Add support for the Transfer Funds API

[alfaproject]

I guess it would be good to add support for the Transfer Funds API: https://docs.opskins.com/public/en.html#ITransactions_TransferFunds_v1

[luuktap]

OPSkinsAPI.prototype.TransferFunds = function(params, callback) { this._requireKey(); this.post("ITransactions", "TransferFunds", 1, params, function(err, res) { if (err) { callback(err); return; } callback(null, res); }); } ;

This code can be added to the @opskins/api/interfaces/ITransactions.js file and it will add this functionality.
Ofcourse it would be very nice to have this in the module by default, @DoctorMcKay ;)

[james-gould]

@luukonetap why not make a PR?

[luuktap]

@luukonetap why not make a PR?

To be honest, I don't use Github much but I'll take a look.