support belongsToMany relationship

[raftalks]

This PR introduces belongsToMany relationship.
Example usage syntax:

Team:
    name: string
    description: longtext
    relationships: 
        belongsToMany:User

User:
    username: string
    relationships: 
        belongsToMany:Team

Generates migration pivot table and adds relationship to the model

[jasonmccreary]

Thanks for this. I'll review this evening and add some tests as I will need to do so before accepting.

[jasonmccreary]

Thank you for the start. Looks like another contributor has used this and added the missing tests. So I have merged your work through #113.