Reorganize supply drop

My fixes for the defaults fixed Rake but starting Rails via 'rails s' still had an issue. Therefore, I completely reorganized supply_drop to emulate other cap3 gems. Examples are:
https://github.com/seuros/capistrano-puma
https://github.com/capistrano/rails

This does break existing setups since the require has changed from 'supply_drop' to 'capistrano/supply_drop' but I figured since we are in a 1.0.0 beta it's okay to make breaking changes. Plus we need this gem to play nicely with others :)