Fix belongsToMany example

I noticed that the user model is doing a `belongsToMany(User)` and I
figured you probably meant `belongsToMany(Account)`.
Merge pull request #16 from jgable/fixBelongsToManyExample

Fix belongsToMany example
minor doc fix on when version listed
Merge branch 'master' of github.com:tgriesser/bookshelf

* 'master' of github.com:tgriesser/bookshelf:
  Fix belongsToMany example