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
any way to preserve this for the `yo` and `slc` usage?