Improve configuration loading and add ability to merge configurations

[kke]

Fixes #68
Would help solve kontena/mortar#86

• Adds K8s::Client.autoconfig which attempts to build or load a configuration using the information available from the environment or local filesystem , see below
• Enable namespace: parameter in K8s::Client.in_cluster_config
• Make all attributes of K8s::Config optional and initialize empty objects as default values
• Add K8s::Config.merge that follows the rules specified in kube docs
• Add K8s::Config.from_kubeconfig_env that parses config file paths from KUBECONFIG and merges the configurations together if it specifies multiple paths (separated by : as documented in kube docs)
• Add K8s::Config.build that can be used to build a minimal working configuration by passing in at least kubernetes server address, certificate authority data and an access token.
• Improves yardocs for some methods

Autoconfig look-up order:

• KUBE_TOKEN, KUBE_CA, KUBE_SERVER environment variables (Using K8s::Config.build)
• KUBECONFIG environment variable (Using K8s::Config.from_kubeconfig_env)
• $HOME/.kube/config file (Using K8s::Config.load_file)
• In cluster configuration (when running inside a kube cluster) (Using K8s::Transport.in_cluster_config)

Breaking changes:

• in_cluster_config now raises if the environment variables are not set (it would have raised anyway if the hardcoded paths were not readable)
• K8s::Config.new does not raise when initialized without previously required parameters (does not break if it was used in a way that worked previously)

[jakolehm]

LGTM

[jakolehm]

@jnummelin PTAL