Add support to fetch config file from network

Closes #1209

Improves kubecfg to let you use config files fetched via http(s) to do the following:
```
$ kubecfg -c "http://kubernetes-tutorials.com/tutorial1/pod.json" create /pods
$ kubecfg -c "http://mybuild-server.com/replicationController.json" create /replicationControllers
```
This lets terminals that use kubecfg not require write access to build files to create a pod, etc.

We should really get in the habit of passing around io.Reader for this sort of thing.