* Adds update option to plugin command
Fixes issues/2385 - helm install silently updates the plugin, if it pre-existed
* Added tests for new methods for plugin update
* Updated docs
* Updated review comments :)
* Return error exit code when there is error
Add plugin management subcommands for installing and removing plugins
to `$HELM_HOST/plugins`.
Install accepts a vcs url or a local directory.
```
$ helm plugin install http://github.com/adamreese/helm-env
Installed plugin: env
$ helm plugin list
NAME VERSION DESCRIPTION
env 0.1.0 Print out the helm environment.
$ helm plugin remove env
Removed plugin: env
```
closes#1977
