global npm packages (Fixes #39)

Fixes #39

I implemented a minimalist solution for global npm packages:

```ndm <cmd> -g <package>```

```bash
ndm generate -g <package>
ndm interview -g <package>
ndm start -g <package>
ndm remove -g <package>
#etc ...
```

If you want to see this interface in action, I'm using it for my project [npmfs](https://github.com/seanewest/npmfs/tree/ndm_global)

I see a couple of benefits in doing it this way:
* looks just like ```npm <cmd> -g <package>``` (but it can't do ```ndm -g <cmd> <package>```)
* avoids name collision with ```npm start <local-service>```
* relatively simple change

There are three main things currently missing (at least from how this feature was discussed):
* the syntax ```ndm <cmd> <package>``` is not supported
* ```ndm start <package>``` currently runs just like it does for local ndm wrappers and does not shortcut the interview -> generate -> start process.
* It uses another flag ```--prefix``` which defaults to ```/usr/local``` to determine the location of npm modules, rather than programatically asking npm. The npm module requires an async [load](https://www.npmjs.org/doc/README.html#using-npm-programmatically) method which I was having a hard time working in with the currently synchronous initialization phase. 

I'd like to hear your feedback and suggestions.