BIG COMMIT -- Parser improvements!

Changes:
1) methods are tightly bound to objects
  -- The dot ('.') method separator bind the method to the object.
     Example: 6+4 / 3+2   ==  (6+4) / (3+2)
	      "hei".uc + "YOU".lc; 		# HEIyou

   -- The arrow ('->') method separator bind the method to the result of the most leftside expression.
     Example: "a".uc + "b".uc -> say;   # says "AB"

2) improved the execution support;
3) many potential bug-fixes;
4) the parser became more strict about the syntax; for instance, there can't be two or more consecutive commas separating an expression.
5) two consecutive empty lines (or with comments) means the end of the previous statement;

Due to minor syntax change, the .sf files has been corrected and improved.