Allow users to create and use custom smart packages.

This pull request adds the following commands to the CLT -

- `meteor package <name>` - Create a smart package named `<name>` from the `package-skel` skeleton. In order to use the smart package you need to register it with `meteor register <name>`. You can pass an absolute or relative path.

- `meteor register <name>` - Register a custom smart package for use in your projects. This allows you to use the smart package the way you would use any other package (i.e `meteor add <name>`). This command can also be used to register and use packages from atmosphere, without having to install meteorite. With `--undo` (`meteor register --undo <name>`), it will un-register a registered custom smart package.

I've also added a `--registered` option to `meteor list` (`meteor list --registered`). This will list all registered custom smart packages.

As of now, I've only added the extra commands to the docs. The smart package API is still undocumented.