HasMany exists should use internal findById

This limits the scope correctly, taking polymorphics into account.

(the foreign key check is actually obsolete I think)
Merge pull request #225 from fabien/fix/has-many-exists

HasMany exists should use internal findById
Merge pull request #224 from fabien/fix/ldl-polymorphic

Properly handle LDL for polymorphic relations
Fix ModelDefinition toJSON bug
Add ability to apply a plugin multiple times from LDL
Merge branch 'development' of https://github.com/fabien/loopback-datasource-juggler into development