fix retain cycle between PXStylesheetParser and PSStylesheetLexer

fix memory leak detectable by Instruments by using weak delegate reference
```
#	Type	Details	Graph
24	 PXStylesheetParser - 2 nodes	Simple Cycle	 
 	  <PXStylesheetParser 0x118e4a460>	 	 
 	  lexer_	PXStylesheetLexer*  [Strong]	 
 	  <PXStylesheetLexer 0x118e4a4a0>	 	 
 	  _delegate	id <PXStylesheetLexerDelegate>  [Strong]	 
 	  <PXStylesheetParser 0x118e4a460>	 	 
```