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