Unify the clients

[pilhuhn]

Currently we have inventoryclient, metricsclient, alertclient and so on, where the user needs to instantiate each client separately.

We need to have one hawkular-client that the gets the base path of hawkular and/or the metrics server and which then internally creates those client objects so that the user can do something like

client = Hawkular.client.new(creds:..., base_path:..., options:...) 
inv_client = client.inventory
inv_client.list_feeds
alert_client = client.alerts
alert_client.create_trigger 

and so on.