2/13: emw4:
- added ` to Scanner.java
- added <|, `, and |> to Token.java
- added parsing code to JavacParser.java
- added {Bracket,Escape}{Expr,Stat} classes to JCTree.java
- added interfaces for same as com.sun.source.tree.* files
- added visitXXX methods to JCTree.Visitor

