Added bzip2() and isBzipped()

Hi Henrik,

Thanks for the very useful R.utils package. I would like to propose adding `bzip2()`, analogous to `gzip()`, and `isBzipped()`, analogous to `isGzipped()`, to R.utils. This pull request implements these, mimicking the analogous gzip behaviour as closely as possible. 

The changes are as follows (including questions, which are mostly related to coding style):

1. I bumped the version number, however, I'm not sure how you assign these.
2. I renamed `R/bunzip2.R` to `R/bzip2.R` to mimic `R/gzip.R` and better reflect the file's new contents.
3. How do you feel about the name `isBzipped()`? It's missing the "2", but `isBzip2ed()` is clunky. Another alternative is `isBzip2()`, although this is missing the "-ed"?
4. I wasn't sure whether it was appropriate to list my name as an author in the `man/bzip2.Rd` file and as a contributor to the package in `DESCRIPTION` - I would be happy with a simple acknowledgement in `NEWS` if you incorporate these changes into R.utils. In any case, I wasn't sure of the syntax for adding my name to the author field with your automated documentation system.

Thanks,
Pete