Gemify, works almost :gem:
Introduce 'configuration file' in yaml format

- Add Retrobot::Config to manage configurations
- Remove user defined global variables and replaced to use
  Retrobot::Config
- add `--config file` option to use configuration file

--

- Implemention notes:

  - Config#load_env and load_yaml_file is called in init_configuration
    because we should control order of loading variables
    (file -> env -> argv)
Add example configuration file
[refactor] Add constant GEM_ROOT
Load config from given path, pwd, or GEM_ROOT
[refactor] extract same logic to method
Load tweets.csv from given path or pwd or gem_root
Correct indent
Remove consumer_key/secret from global
Remove unneed FIXME
Verify credentials faster
Remove unneeded begin-end clause
Give ARGV from constructor
Configurable retryable.gem variables
Add README
Merge pull request #5 from sorah/revolution

Revolution (gemify, add config yml)