install: Add support for installing from an SVN URL

[HenrikBengtsson]

Add support for installing from a URL that hold a package directory, e.g.

source("http://callr.org/install#https://<username>:<pwd>@hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/codetoolsBioC/")

[HenrikBengtsson]

Would require a downloadFiles(url, recursive=TRUE) method.

[HenrikBengtsson]

Can be done using devtools, e.g.

devtools::install_svn("https://hedgehog.fhcrc.org/bioconductor", 
                      subdir="madman/Rpacks/codetoolsBioC")

Some care has to be taken for trunk and branches.