Added ability to configure name of SDK which should be sent along with every request as pnsdk query parameter.

New configuration option which can be passed to constructor is: sdkName.

Basic usage

let client = new PubNub({
    subscribeKey: 'mySubKey', 
    publishKey: 'myPublishKey', 
    uuid: 'myUUID', 
    sdkName: 'custom-sdk/1.0.0' 
});