Implemented polymorphic hasMany
Implemented polymorphic hasAndBelongsToMany
Implemented polymorphic hasOne

Signed-off-by: Fabien Franzen <info@atelierfabien.be>
Minor cleanup
More hasAndBelongsToMany tests
Implemented polymorphic hasMany through inverse

Added option invert: true to enable inverse polymorphic hasMany
relations.

Fixed missing fk1 in query of HasManyThrough.prototype.exists.
Minor tweaks; pass-through properties/scope for hasAndBelongsToMany
typeTo => discriminator
polymorphics can now be declared using object
Updated remaining relations to use polymorphicParams
Merge branch 'feature/polymorphic-rel' of github.com:fabien/loopback-datasource-juggler into fabien-feature/polymorphic-rel
Fix test cases
Merge branch 'fabien-feature/polymorphic-rel'