Add install instructions, and seed task creates an admin
Add "dependent_supply_of_carrier" to Link

Like "primary_demand_of" and "sustainability_share", it takes the value
of the right-hand (supplier) converter, reducing that value according to
the slot conversion and link share.
Merge pull request #664 from quintel/link-dependent-supply

Add "dependent_supply_of_carrier" to Link
Remove old, unused dependencies

Term::ANSIColor and Highline, which are used only by the bulk_update
task, have been removed from the Gemfile, and will now propmt the user
to install the gems manually if they aren't available.
Should have known better than to second-guess Bundler