Bioconductor dependencies from package description

This will make the support for Bioconductor package development more similar to CRAN. I have not yet included installing the devel version of R (nightly snapshot), which is what some developers use for CRAN and sometimes Bioconductor; we can do that in the future.
Interesting, I didn't know about `${something:-somethingElse}` or `${something-somethingElse}`. It took me a while to find that it's called [parameter substitution](http://tldp.org/LDP/abs/html/parameter-substitution.html) in bash.