new tree node, fixes, ASTBuilder

- added GIMMEH node
- added exceptions package
- TreeFunctionParameter is now a variable
- TreeValue has now Expression-type value
ASTBuilder implementations, grammar fixes, TreeValue fix, added TypedValue interface, antlr4.1 runtime

- antlr 4.0 -> 4.1
- constant expressions in grammar now have own rule
- DIV and SUB expressions now have own rules
- implemented visitValue, visitExpr(for IDs), visitAssstat, visitAddexpr
- fixed wrong return type in visitVarDecl
- fixed extra visitFile call inside visitFile
- lhs in AssignStmt is now a variable, rather than a plain value
- TreeConstant now holds a real value from parse tree