TextAreaItem - allowing characters \n and \"

This trivial patch allow characters ```\n``` and ```"``` in property values.
I also thing that adding support of newlines in multiline textarea items is good idea.
(DMR properties works correctly with it, only web console have problem with it.)

**Before this patch cause character ```"``` poorly trackable UmbrellaException:**
```
[ERROR] 2014-04-16 10:20:07,512 [FATAL] Uncaught Exception:
[ERROR] com.google.gwt.event.shared.UmbrellaException: Exception caught: Could not parse payload
[ERROR] Caused by: java.lang.RuntimeException: Could not parse payload
[ERROR] Caused by: org.json.JSONException: Expected a ',' or '}' at character 68
[ERROR]     at org.json.JSONTokener.syntaxError(JSONTokener.java:410)
```
In WildFly web console visible only as not saving without any error message. (Only exception in GWT debug)