Proposal: Better assistance to help first time users of fsi

fix #92

with FSI:
help command is `#help;;`
quit command is `#quit;;`

but first time users maybe try instead `help;;` `quit;;` `exit;;` `#exit;;`

This commit add eval of:
* `quit;;` or `exit;;` ( without # ) show, after eval result, the
howto quit help message
* `help;;` show, after eval result, the help message
* `#exit;;` show, after invalid directive message, the howto quit
message

On mono, the quit help message write `Ctrl-D` instead of `Ctrl-Z`

examples:
```
> quit;;

  quit;;
  ^^^^

stdin(1,1): error FS0039: The value or constructor 'quit' is not defined
Use #quit;; or Ctrl-Z to exit
```
```
> exit;;
val it : (int -> 'a) = <fun:clo@2>
Use #quit;; or Ctrl-Z to exit
```
```
> #exit;;
Invalid directive '#exit '
Use #quit;; or Ctrl-Z to exit
```
```
> help;;

  help;;
  ^^^^

stdin(4,1): error FS0039: The value or constructor 'help' is not defined

  F# Interactive directives:

    #r "file.dll";;        Reference (dynamically load) the given DLL
    #I "path";;            Add the given search path for referenced DLLs
    #load "file.fs" ...;;  Load the given file(s) as if compiled and referenced
    #time ["on"|"off"];;   Toggle timing on/off
    #help;;                Display help
    #quit;;                Exit

  F# Interactive command line options:

      See 'fsi --help' for options
```