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
Bump version
Merge branch 'release/2.1.0' into production
Merge tag 'v2.1.0'

2.1.0
Fix a regression where undefined id should not match any record
Bump version
Merge branch 'release/2.1.1' into production
Merge tag 'v2.1.1'

2.1.1
Implemented embedsMany relation
Added validation for embedded items (optional)
Minor fix
Test build of embedsMany
Require unique ids for embedded items
typeTo => discriminator