Add tests for cross-database relationships

[kmalakoff]

Ensure join table locations are deterministic

[forivall]

Regarding the location of the join table, i would favour an approach that there is some kind of defined preference based on backends, (e.g. prefer creating the join table on sql instead of mongo; memory > sql > mongo) and if the tables use the same backend but different databases on that backend, either use an arbitrary but deterministic preference (like alphabetical order), or throw an error.
In either case, the join table location should be able to be manually defined like so

class Foo extends Backbone.Model
  schema:
    bars: -> ['hasMany', Bar, join_on_this_database: true]  # choose a better key name
  sync: FooSync('foo')

class Bar extends Backbone.Model
  schema:
    foos: -> ['hasMany', Foo]
  sync: BarSync('bar')