Feature/slc integration

/to @ritch 
/cc @bajtos 

This PR is peer to https://github.com/strongloop/strong-cli/pull/154

- Update the `USAGE` to reflect `slc loopback:*`
- Remove the peer dependency to `yo` (even with global installation, the expectation is `yo` is there already)
- Exports `yeoman-generator` module so that `strong-cli` doesn't depend on it directly
> Remove the peer dependency to yo (even with global installation, the expectation is yo is there already)

I am not confident this is a good change. With this change in place, when you install a fresh instance of Node and run `npm install -g generator-loopback`, you will end up with a setup that does not work.

The peer dependency was added by `yo generator`, AFAIK all other generators include it in package.json.