Added preliminar support for legend

This changes a couple of height/width dimensions and adds a dummy legend. You will need the latest version of rCharts. You will need to fix this part of the code to get real legends. Replace colors with the user input colors and labels with the species names.

```coffee
output$map_rcharts <- renderMap({  
  imap = gbifmap2(input = rcharts_data(), input$provider)
  imap$legend(
    position = 'bottomright',
    colors = c('red', 'blue', 'green'),
    labels = c('Red', 'Blue', 'Green')
  )
  imap
})
```