Optional publish readme argument

I have created an example of providing the readme file as a list to the `publish` method; e.g. `obj$publish(readme = list("My visualisation"))`.

Another example:
```
map4 = Leaflet$new()
map4$setView(29.6779, -95.4379, 10)
map4$tileLayer("http://{s}.tile.cloudmade.com/BC9A493B41014CAABB98F0471D759707/997/256/{z}/{x}/{y}.png")
data(crime, package = 'ggmap')
dat <- head(crime)[,c('lat', 'lon', 'offense')]
names(dat) <- c('lat', 'lng', 'offense')
map4$geocsv(dat)
map4$publish(readme = list(
    "Leaflet visualisation" = "This map was created using [rCharts](http://github.com/ramnathv), [Leaflet](http://leafletjs.com/) and [Leaflet-geoCSV](https://github.com/joker-x/Leaflet.geoCSV)", 
    list("Lorem Ipsum 1" = "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book.", 
         list("Lorem Ipsum 2" = "Contrary to popular belief, Lorem Ipsum is not simply random text. It has roots in a piece of classical Latin literature from 45 BC, making it over 2000 years old.")
)))
```

=> [http://pagist.github.io/?5553827](http://pagist.github.io/?5553827)

In your Leaflet gist example you also provide the source code; will this be implemented as an argument as well, e.g. `obj$publish(readme = list("My visualisation"), code = T)`?

I dunno if this is the preferred solution, but I put it up here for discussion.