Capistrano V3 Support.

This patch adds support for capistrano V3.  No existing files were modified.  Just new V3 specific files added.

To get it working (assuming bundler is being used) add the following to Capfile:

```
require 'sidekiq/capistrano3'
```

And this to config/deploy.rb:

```
SSHKit.config.command_map[:sidekiq] = "bundle exec sidekiq"
SSHKit.config.command_map[:sidekiqctl] = "bundle exec sidekiqctl"
```

The above works around an issue with SSHKit that prevents it from changing to the correct directory if the first string of the command to be executed contains spaces.  Not ideal, but it's life for now.

I can update the Wiki with links/instructions once you merge it if you'd like.