Cloud drive

# Summary

- Create table `cloud_drives`
- Make `Google::Service` as a fully fledged service to handle Google Drive API requests

# Google drive service

example command:
```ruby
s = Google::Service.new
s.initialize_drive_client(CloudDrive.first)

# create directory
r = s.create_directory('test drive', 'test create a folder', 'root')
# get files under 'root directory'
r = s.get_files('root')
# get files (or folders) containing string test
r = s.get_files_by_title('test')
# get a file by its id
r = s.get_file_by_id('0B3xN9bzsxMKfMUdiVm1DZVZGSkE')
# move files from root to the given folder
r = s.move_file('0B3xN9bzsxMKfMUdiVm1DZVZGSkE', 'root', '0B3xN9bzsxMKfNzU1UXZKQnA4QjQ')
```
Please change from if not credential.nil? to unless credentials.nil?