Support for part-of-speech tags; additional examples

This update adds a way to use the part-of-speech tags documented in the [Ngram syntax](http://books.google.com/ngrams/info), as well as a few examples, including this one [by Ben Zimmer](http://languagelog.ldc.upenn.edu/nll/?p=4979):

![rplot](https://f.cloud.github.com/assets/322533/830339/65b76bf6-f146-11e2-9e52-e9f95b3f9deb.png)

I've also added a little security to make sure that `ggram` receives a vector characters for phrases (sending it a data frame will crash your R session).

It would probably be a good idea to have the [Ngram syntax](http://books.google.com/ngrams/info) page cited somewhere. I can submit a revision of the README file for the package, if you are okay with it.