allow user specified build/installation order for R packages

[russHyde]

R packages may depend on each other for installation.

Typically, copied packages may depend on cloned packages and project-specific package may depend on either copied or cloned packages.

User should be able to configure which gets built / installed in which order.

[russHyde]

Could just parse clone_these_repos, copy_these_dirs and build / install in order (filtered by presence in lib/[cloned|copied]_packages). Then install a project-specific package, if required.

[russHyde]

We no longer use lib/[cloned|copied]_packages (as of #91 ), we use lib/remote and lib/local; anything in lib/remote is built/installed before anything in lib/local. Have not yet implemented an installation ordering script/config