Add the ability to easily disable skills-client

[rmmayo]

Add new optional enabled parameter to the configure method:

 this.SkillsClient.SkillsConfiguration.configure({
           serviceUrl: 'http://localhost:8080',
           projectId: 'your-project-id',
           authenticator: '<auth>',
           enabled: false
        });

when disabled the skills-client library should remain dormant, and not attempt auth, network communication, skill reporting, etc.

Please Note: enabled will default to true

[sudo-may]

PR: #254