Add Gemfile

[stjhimy]

Add a Gemfile for easily bundle install before starting development.
It also checks out from the repository the Gemfile.lock as explained here http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

[janfri]

I don't use bundler. But for you out there it's merged in mini_exiftool 2.7.6. ;-)