JSON Import API

API methods :
```
Realm.createObjectFromJson(Class realmObject, JSONObject json)
Realm.createAllFromJson(Class realmObject, JSONArray json)

Realm.createObjectFromJson(Class realmObject, String json)
Realm.createAllFromJson(Class realmObject, String json)

// Uses JsonReader for stream parsing. Only for API 11+
Realm.createObjectFromJson(Class realmObject, InputStream json);
Realm.createAllFromJson(Class realmObject, InputStream json);
```

A few comments:

- Import using InputStream only works for API 11+. Methods are annotated as such.

- JSON property names need to match java variabel names.

- Import API does not work in standard Java. JSON is only supported from Java 7, and with a different API. 

- If import fails, everything up to that point is put in the database (if Realm.commitTransaction() is called). We need a Realm.rollback() to change this behavior.

- functions like createOrUpdateFromJson() would be natural extensions but awaits primary key support.
We could possible include it in our own namespace. We got these kinds of checks all over the place. But I would rather have that in another PR.