Modify to gem repository

Run following commands;
  bundle gem chatwork_cli
  mv chatwork_cli/* ./
  mv chatwork_cli.gemspec chatwork-cli.gemspec
  mv lib/chatwork_cli.rb lib/chatwork-cli.rb
  mv lib/chatwork_cli lib/chatwork-cli

Move "chatwork_cli/.gitignore" content to ".gitignore".

Run "rm -rf ./chatwork_cli".

Fix default README by original README.
Fix chatwork-cli.gemspec

Replace "chatwork_cli" to "chatwork-cli".
Remove executable permission
Fix filepath, filename and directory structure
Fix lib/chatwork-cli.rb

Replace "chatwork_cli" to "chatwork-cli".
Create executable file
Write summary and description
Fix bug

Avoid the following error;

  lib/chatwork-cli/get-messages.rb:10:in `gets': No such file or
  directory @ rb_sysopen - get-messages (Errno::ENOENT)
Add Usage and Development section to README