Allow end users to create a blank plugin project with the 'createplugin' command

I thought it'd be worthwhile to write this up, since I'll be making plugins for spreads.

The downside is that this is against upstream/master, so if spreads.plugin changes as per other open issues (#32, #33, #35, #37) this will be out of date.

This branch makes a project directory fine, but its unknown whether the project directory can make a usable plugin!

There's no need to merge this just to see how well it works! Just:
$ pip install git+git://github.com/matti-kariluoma/spreads.git@plugin-boilerplate
$ spread --verbose createplugin DevicePlugin MyCamera
$ cat mycamera/README.txt
$ sudo pip install mycamera/
$ sudo pip uninstall spreadsplug_mycamera