S3 method for function class

I have added basic method for `function` class.
For now it just renders the body and adds function name if specified by `add.name` param.
For example,

```
> pander(splitLine)
function (x, max.width = panderOptions("table.split.cells"),  
    use.hyphening = FALSE)  
{ 
    if (!is.character(x) || !is.null(dim(x)) || length(x) !=  
        1)  
        return(x) 
    if (suppressWarnings(!is.na(as.numeric(x))))  
        return(x) 
    if (is.infinite(max.width))  
        max.width <- .Machine$integer.max 
    if (use.hyphening && !require(koRpus))  
        use.hyphening <- FALSE 
    hyphen_f <- function(s) hyphen(s, hyph.pattern = "en.us",  
        quiet = TRUE)@hyphen[1, 2] 
    .Call("pander_splitLine_cpp", PACKAGE = "pander", x, max.width,  
        use.hyphening, hyphen_f) 
} 
```
Name added by `add.name` param.
```
> pander(splitLine, add.name = T)
splitLine  <- function (x, max.width = panderOptions("table.split.cells"),  
    use.hyphening = FALSE)  
{ 
    if (!is.character(x) || !is.null(dim(x)) || length(x) !=  
        1)  
        return(x) 
    if (suppressWarnings(!is.na(as.numeric(x))))  
        return(x) 
    if (is.infinite(max.width))  
        max.width <- .Machine$integer.max 
    if (use.hyphening && !require(koRpus))  
        use.hyphening <- FALSE 
    hyphen_f <- function(s) hyphen(s, hyph.pattern = "en.us",  
        quiet = TRUE)@hyphen[1, 2] 
    .Call("pander_splitLine_cpp", PACKAGE = "pander", x, max.width,  
        use.hyphening, hyphen_f) 
} 
```

What do you think?
One problem that I realized though. When converting to pdf using pandoc all tabulations are being discarded. How can that be avoided?