import devise and user model

For the like/dislike button and to allow users
to manage their snippets we will need users.
add like/dislike

Logged in user can now like/dislike an snippet.
move options in own partial

to make maintaining of the available options easier
make like/dislike with ajax
move loading image to application.html

So there is a loading spinner available on all pages
and their ajax functions
add titles to option-links
show liker/disliker in show-view
add error handling for like/dislike
Merge pull request #22 from darinkes/like_dislike

Like dislike