(maint) Add untar_into method

This commit adds a simple method that shells out to tar to unpack a tarball
into a target directory. Basic sanity checks are present, but this assumes a
tar that can gunzip and untar at once. Arbitrary options can also be passed to
the method.
(maint) Add fetch_uri method to Pkg::Util::Net

This commit adds a simple utility method to fetch and download a uri to a
location on disk. It is pure ruby and as such is more portable than curl or
wget.