Third possibility of installing scripts

Added ability to add scripts that don't belong to S3. This can work fine as
long as there's only unidirectional dependencies. Script's installed this
way will produce a warning however.

NOTE: scripts installed this way can't be uninstalled with
s3.uninstall_all.sh, which ipso-facto will break any outside script still
linguering in ~/bin (or /usr/local/bin).

Category: development