Fix/lua no funcs known (#15)

* lua fixes:

- fix lua function calling, didn't work in the last
releases (regression)
- add lua funcs which don't modify the stack (for converters etc)
- added better lua examples

rpn.pod

@@ -273,8 +273,13 @@ function name
 
 =item *
 
-number of arguments expected (1,2 or -1 allowed), -1 means batch
-mode.
+number of arguments expected (see below)
+
+Number of expected arguments can be:
+
+    - 0: expect 1 argument but do NOT modify the stack
+    - 1-n: do a singular calculation
+    - -1: batch mode work with all numbers on the stack 
 
 =item *