[Dependencies] Update to Athena-Core 1.2.0 and add rapidjson 0.11
Add a function to serialize a Component into a rapidjson object
Add a function to deserialize a Component from a rapidjson object
Add a function to serialize a Component into a JSON string
Add a function to deserialize a Component from a JSON string
Prettifying
Add a function to serialize an Entity into a rapidjson object
Add a 'getChild()' method to the 'Entity' class
Add a function to deserialize an Entity from a rapidjson object
Add a function to serialize an Entity into a JSON string
[Unittests] All the JSON string-related tests now load data from a file