Setup default_environment from .slashdeploy.yml

[russellballestrini]

Also alter environments message to show which environment is default

Also updates the /docs page with many of the new features we introduced.

modified:   app/models/environment.rb
modified:   app/models/repository.rb
modified:   app/views/messages/environments.text.erb
modified:   lib/slashdeploy/config.rb
modified:   spec/features/commands_spec.rb
modified:   spec/models/repository_spec.rb

[russellballestrini]

@bmarini I'm in agreement. Right now SlashDeploy keeps this "default_environment" in the database. This simply allows a repo owner to optionally set a default in .slashdeploy.yml.

Previously I think SlashDeploy would automatically set the first created environment as default and there was no way to adjust this. Now SlashDeploy does not setup a default_environment unless explicitly set in .slashdeploy.yml