Add neq operator support for memory connector

See https://github.com/strongloop/loopback-datasource-juggler/issues/265
Added a test case for neq
Isolate transient schema helper
Fix #283
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.
Fix multi-property validation definitions

Fixes #291, as introduced by me ...
https://github.com/strongloop/loopback-datasource-juggler/commit/6fae8c7
27759256157bf7ccd82e04ee372a59088
Merge pull request #292 from fabien/fix/refactor-validations

Fix multi-property validation definitions
Merge pull request #287 from fabien/fix/tiny

Export RelationClasses
Merge branch 'master' of github.com:strongloop/loopback-datasource-juggler into 2.0-patched