Add guzzle client as class member

We are using our charge app behind a proxy manager and need to set proxy settings for guzzle client. Moved client instance from ChargeBee\ChargeBee\Guzzle::request to a new class member as $client and added public setter / private getter. If a custom client is not set getClient initializing a new Guzzle\Client

Example usage will be like:

\ChargeBee\ChargeBee\Guzzle::setClient(new GuzzleHttp\Client([
    'proxy' => [
        'http'  => 'http://localhost:8125', // Use this proxy with "http"
        'https' => 'http://localhost:9124', // Use this proxy with "https"
    ]
]));

chargebee / chargebee-php

Add guzzle client as class member #43