Bundler fixes

This splits the bundler-commands into seperate lines, to allow easier adding, deleting and therefore merging of new PRs.

fixes #1817
fixes #1825
fixes #2779
fixes #2200
fixes #2561
fixes #2076 
fixes #1818 
fixes #2885

already fixed, but issue still open:
closes #2695

not needed:
closes #2074 

`spring` is added.
`pry` is added.
`sidekiq` is added.
`ruby` is removed.
`berks` is removed.
`foreman` is removed.
`spin` is removed.

If a binstub in `./bin/` exists, it is called instead of `bundle exec <gem>`.

Added functionality to include additional executables with wrapper: BUNDLED_COMMANDS.

A README.md file is added to document this plugin. Added documentation on how BUNDLED_COMMANDS and UNBUNDLED_COMMANDS works.