new AsyncGrammar()
        loads the default grammar asynchronously using a WebWorker
NOTE usage of the grammar is also async:
     calls on 
    
    
    
    
    
    
    
    
interpret(text, callback) are re-directed to the WebWorker
    Methods
- 
    
init()
 - 
    
    
    Initialize a grammar to be loaded & executed asynchronously After