fix labels

The discrete scale needs labels to be manually set to preserve upper case in the keywords. Here's an example:

    ggram(c("USSR", "Russia"),
          year_start = 1900,
          corpus = "eng_gb_2012", 
          geom = "area",
          geom_options = list(position = "stack"),
          ignore.case = TRUE) + 
      aes_string(fill = "Phrase") + 
      scale_x_continuous(breaks = 1900 + 10*0:11) +
      scale_fill_discrete("", labels = c("USSR", "Russia"))

![](http://i.imgur.com/ALCfdrs.png)
Merge pull request #9 from briatte/patch-2

fix labels