Backport to dev_tools the tooling added to solidus_static_content

[elia]

solidus_static_content had a number of recent changes devoted to easing the development experience. We want to bring in anything that's both useful and generic enough to be good for all extensions.

What to do

Make a list of things that should be ported here, open PRs accordingly.

• junit formatter for better circleci output (Include rspec_junit_formatter #6)
• bin/rails bin/rake bin/setup bin/update bin/console scripts, possibly also bin/server and bin/release (Generate bin files #35, original PR)
• standardize the extension structure to use the full extension name as main namespace and bundlers gemspec defaults (original PR)
• move from selenium to apparition + puma for feature specs (original PR, Move from selenium to apparition for JS feature specs #22)