Making ggram accept any geom

These changes make `ggram` accept any ggplot2 geom. The example below uses `step`.

![rplot](https://f.cloud.github.com/assets/322533/829106/0788b694-f0c8-11e2-81b7-94e3fe9beeae.png)

    ggram(c("cancer", "fumer", "cigarette"),
          year_start = 1900,
          corpus = "fre_2012", 
          geom = "step")

Two examples to illustrate this change have been added to the documentation.