AGPUSH-404 "Add datastore element to WildFly configuration"

To test you can start with a clean standalone.xml and install the module, then run the ```wildfly.config``` script (see wildfly-module/README.md for details).

To try different datastores, update the simplepush configuration element in standalone.xml:
```
<datastore>
    <redis host="localhost" port="6379"/>
</datastore>
    
<datastore>
    <couchdb url="http://127.0.0.1:5984" database-name="simplepush"/>
</datastore>
    
<datastore>
    <in-memory/>
</datastore>
```