Added ability to configure kubectl via kubeconfig rather than point a…

[timcrider]

…t an endpoint.

• Kubeconfig is preferred if it exists
• Currently endpoint and kubeconfig are mutually exclusive at this time
• Backwards compatible

var k = require('k8s');

// Configure using endpoint
var kube = k.kubectl({
    endpoint: 'http://my-cluster'
    ,binary: '/bin/kubectl'
    ,version: 'v1'
});

// Configure using kubeconfig
var kube = k.kubectl({
    binary: '/bin/kubectl'
    ,kubeconfig: '/etc/cluster1.yaml'
    ,version: 'v1'
});