Add neq operator support for memory connector

See https://github.com/strongloop/loopback-datasource-juggler/issues/265
Added a test case for neq
Polymorphic lookup from all registered dataSources

Polymorphic model lookup was previously limited to the same dataSource
as the modelFrom model, which turns out to be too restrictive. This was
uncovered by the use of a Transient model, not being able to lookup a
PersistedModel.
Re-use modelBuilder - correctly fixes lookup
getTransientSchema => getTransientDataSource
Remove legacy Schema references
Merge pull request #286 from fabien/fix/polymorphic-transient

getTransientDataSource helper + embedsMany/polymorphic test case fix
Simplify the id lookup
Bump version
Merge pull request #266 from strongloop/feature/fix-issue-265

Add neq operator support for memory connector
Merge branch 'release/2.8.0' into production
Merge tag 'v2.8.0'

2.8.0
Export RelationClasses

Without this being exported, it was impossible to define custom
relation types. However, there are still limitations - not all helper
methods are public to really handle this scenario.