add license
turn fourchette into a gem
moving remaining dev & test gems to gemspec
ignore LoadError on pry in dev

Pry helps us when hacking on Fourchette, but otherwise, we don’t care.
If Fourchette is used as a gem in another app, in dev, it was trying to
load Pry even if it might not be present. This is a awful hack but it
works for now.
run rackup in Bundle's context
expose rake tasks to the outside world
update README to reflect new reality as a gem
remove puma and Gemfile.lock