#metadata should be more flexible and also output unknown metadata 

With this pull request, all metatags are returned, even unknown ones.

It also fixes pdf_info on Ruby 1.9.2

Beside this, it also does some minor cleanup:
* ~~`Gemfile.lock` is back. This is how it should be :)~~
* Travis CI support - you just need to set up the hook at github
* syntax highlighting in the `README.md`
* Code Climate badge