make it easier to install nwm

I've added a bash script to make nwm easier to install.

It generates nwm.desktop, and has a script to start nwm inside xephyr.

```
./nwm init                #create nwm.desktop, and ~/.nwm/nwm-start
                              #this is called automatically by npm install
sudo ./nwm install    #move nwm.desktop to /usr/share/xsessions/
                              #will log to ~/.nwm/*.log
./nwm xephyr [size]  #start nwm in xephyr
```

It ended up uglier than I had hoped... because that is bash.
I had some problems since node can't see sudo on my coniguration.
but I've worked around it, hopefully in a way that will work for everyone.

Also, modified the scripts in the package.json so it would install for me,
I had to `cd build; make` to get `./build/Release/nwm.node`. I added this to `scripts.postinstall`

cheers,