Keep undefined/null values for the array type

This allows connectors to distinguish between empty array and
undefined/null. For example, mongodb will not override existing array
properties if the value is undefined.
Merge pull request #126 from strongloop/feature/fix-array-update

Keep undefined values for the array type
2.0.0-beta1
Remove unsupported connectors

 - cradle
 - http
 - neo4j
 - riak
Merge branch 'feature/remove-unsupported-connectors' into 2.0

Close #120
Replace connector base with loopback-connector

Remove references to Connector and BaseSQL, connectors should require()
loopback-connector instead of loopback-datasource-juggler.
ModelBuilder: add `prototype.defineValueType`

Add a shortcut for registering a new value type.

The current implementation registers the type in the singleton registry
`ModelBuilder.schemaTypes`.

The API should allow us to to change the implementation to register
the type in the scope of ModelBuilder instance only.