add utility for binding flags and building api server clients

This utility provides one stop shopping for a command line executable to get bind the correct flags, build the client config, and create a working client.  The code for usage reduces to something like this:

	authLoader := &PromptingAuthLoader{os.Stdin}
	clientBuilder := clientcmd.NewClientBuilder(authLoader)
	clientBuilder.BindFlags(cmds.PersistentFlags())
	apiClient, err := clientBuilder.BuildClient()

I made the flags congruent with kubectl and used kubectl as an example spot for usage.  This makes use of the auth config work in #2340 and the Bind and Build approach could be used to unify client creation and flag names.

@smarterclayton You've had an interest in this area.



> On Nov 20, 2014, at 6:46 AM, David Eads <notifications@github.com> wrote:
> 
> In pkg/client/cmd/doc.go:
> 
> > +
> > +Licensed under the Apache License, Version 2.0 (the "License");
> > +you may not use this file except in compliance with the License.
> > +You may obtain a copy of the License at
> > +
> > +    http://www.apache.org/licenses/LICENSE-2.0
> > +
> > +Unless required by applicable law or agreed to in writing, software
> > +distributed under the License is distributed on an "AS IS" BASIS,
> > +WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
> > +See the License for the specific language governing permissions and
> > +limitations under the License.
> > +*/
> > +
> > +/*
> > +Package cmd provides one stop shopping for a command line executable to bind the correct flags,
> "cmd" doesn't convey to me what the docs describe. Any other names?
> 
> clientcmd? That would make the qualified package pkg/client/clientcmd. http does this with net/http/httputil, but I had noted the kubernetes had pkg/kubectl/cmd.
> 
I wouldn't take that as a constraint.  clientcmd sounds fine.

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