[dml-compiler] Added JsonElement to the Builtin Value Types

  * Also using Gson 2.2.4 (bug fix release)
  * See #158
[core-api] Externalization.externalizeObject now supports externalizing JsonElements

  * This allows Backends which rely on externalizeObject to persist data (such
    as Infinispan and OGM)
  * See #158
[jvstm-ojb] Added Support for the new JsonElement Value Type

  * See #158
[core-api] Added Utility Class to convert to/from JsonElements

  * Closes #159
[core-api] Added Tests for JsonConverter
[dml-code-generator] Complex Value Types now externalize to Json

  * Closes #165
[jvstm-ojb] Refactor: Now supporting complex ValueTypes

  * The backend now knows that complex Value Types are serialized to JsonElements,
    and as such, knows that the SQL type for them is that of the JsonElement.
  * Instead of doing manual [de]serialization of ValueTypes, the generated code
    now delegates that responsability to the ValueTypeSerializer
  * Closes #166
Merge pull request #167 from jcarvalho/feature/json-element

Adding Json Support to the Framework