Spring Cleanup

[Tensho]

I'd like to make some obsolete stuff cleanup in the gem. As far as capistrano-sneakers tries to copy common parts from this gem, it will be nice to cleanup here and then copy the code there. It will allow maintainers/contributors to switch between gems much easier.

Major notes

• Remove capistrano 2 files, because it's support was dropped long time ago. Current gemspec holds dependency version 3.9.0.
• Sidekiq works with different signals to quit in 4 (USR1) and 5 (TSTP) version, but current manual signal mode sends only USR1.
• Standard sidekiq binary to control the processes "sidekiqctl" already works with signals, so there is no need to keep a separate branch for enabled "sidekiq_use_signals". Moreover sidekiqctl has very important feature – kill timeout, that allows sidekiq process to be gracefully stopped and, the most important, pidfile removed. For example, for the service restart (stop-start), if deploy script uses only signals, "stop" task just sends the signal and passes execution to the "start" task without waiting of any pidfile deletion. That means deploy script doesn't start the sidekiq due to the process and pidfile existence condition fail ("pid_process_exists?").
• Capistrano/SSHKit deprecated #background method.
• Bump 1.0.0 to make people aware about it's stability and maturity. I guess this gem deserved to be called production-ready ^_^
• [BREAKING CHANGE] If people used custom monit template, they should adjust it to use pid_files variable instead of processes_pids.

Minor notes

• Renamed methods to stress their purpose.
• Small adjustments to README (wiki page requires the changes also)

I may squash commits after review if you wish to keep it in a single commit.

[seuros]

This is awesome. Releasing the version now.