Draft of a help function for method documentation similar to reference classes

Related to #2, I was looking for ways to document methods. The code in this pull request mimics the documentation system of reference classes, where each methods can optionally be documented with a string, given as first line of its body:

```R
TC <- R6Class("test_class",
              public = list(getx = function() { 'Gets x'; private$x },
                            setx = function(x) { 'Sets x'; self$x <- x },
                            one = function() 1,
                            help = R6_help),
              private = list(x = 1)
)
T <- TC$new()
T$help('setx')
# test_class$setx: Sets x
T$help('getx')
# test_class$getx: Gets x
T$help('one')
# test_class$one: No documentation available.
```

The help function could be automatically added to all objects. Would you be interested in including something like this in `R6`? If there is interest on your side, I will happily improve the code until it is fit for being included.