Add fluentd.agent#backup_config method

and call it in #config_write and #config_append
to backup config histories
Add #backup_running_config method

on Fluentd::Agent
Add UI to reuse backup config files
Fix spec to ensure config_file
Fix spec to avoid depending on spec execution order
Remove keyword argument from methods

for Ruby 1.9
Fix to write config file via Fleuntd::Agent
Add UI for running bacukp config
Change to use array.first(5) instead of array[0..4]
Chamge method name #backup_files -> #backup_files_in_old_order

to make its sort function clearer, and Refactor #remove_over_backup_files by using it
Change to use File.join instead of string concatenation

because File.join makes the purpose of code clear and do it safely
Change to use application.yml setting instead of hard coded number
Merge pull request #120 from fluent/config_history

Config history