Keep debugger support only for rubies < 2.0.0
Update Gemfile templates to provide correct debugger
Update debugging guide to use byebug
Isolate debugger related code
Prevent generation of extra whitespace in templates
Minor fixes in the rails debugging guide
Re-review the debugging guide.

Tested on a brand new app to exactly match current byebug's behaviour.
Merge pull request #14646 from deivid-rodriguez/provide_byebug_by_default

Improve debugging support