fix(json parser): fix classcastexception on getChild with json null value
    
return null instead of throwing classcastexception when asking for child with 
json null value
feat(parser): add getNumber on SwaggerParser
    
add a getNumber(name:String):Number method on SwaggerParser.
feat(utils): add Swagger data parser for fine primitive parsing

Add a SwaggerDataParser that encapsulate SwaggerParser and allow t ask for 
primitive types such as Integer, Long, Float, Double for a given field. 
Returns null value if the field is not present.
feat(models): add "models" support (reading only)

Add a "models" model and update the SwaggerReader implementation to parse it.
The parsing is only performed if the parsed swagger version is 1.2 (previous 
versions are not compatible with the model).
Writing the model (with SwaggerWriter) is not supported yet nor the complex 
data type for model object properties.