fix dependencies

[alyssafrazee]

packages "BSgenome.Hsapiens.UCSC.hg19" and "BSgenome.Mmusculus.UCSC.mm10" are REALLY SUPER SUPER slow in installing and aren't always needed. Is there a way to only load main packages when installing the packages and load those bulky ones only if the user needs them? (i.e. doesn't already have an annotation)?

Also, fix README to include all dependencies (not just the "Depends") and while you're at it, fix Depends vs. Imports.

[brianhigh]

This seems to work:

# Conditionally install packages by testing with require() first and only installing if that fails.
source("http://bioconductor.org/biocLite.R")
for (pkg in c("BSgenome.Hsapiens.UCSC.hg19", "BSgenome.Mmusculus.UCSC.mm10"))
{
    require(pkg, character.only = TRUE) || biocLite(pkg)
}