client should be split into two pieces

[notatestuser]

Right now client has methods that rely on this.privateKey and others that take parameters in arguments (such as checkAddress(address)). These separate concerns should be separated out into two classes.

[Johnsavadkuhi]

class Signer { } class BncClient {} , remove setPrivateKey in BncClient

[notatestuser]

@Johnsavadkuhi putting it in a separate class is essentially the same thing in JS (security wise).