Refactor: `like!` :point_right: `like`.

There wasn’t an ‘undestructive’ version of `like` before. So, no necessity of the bang pair.

And even using the Rail’s semantics, in this case, the exception raised by the `save!` method wasn’t being used anywhere.

And for finals: the vote returned by the method was just being used inside a spec. So, let’s fallback to the good’ol `save` and verifying for its truthiness inside the controllers and such.
Tell the user in case his voting up went bad.
Refactor: Isolate the vote definition, so it can be used both by like and dislike methods.
:nail_care:
Remove the save! from the `Vote.dislike` definition.

To more on this, please refer to the `919e912c784a576f40bfbf3f735beed2a194758a` message.
Tell the users if their dislike had gone south.
Merge pull request #68 from ricardovaleriano/bug-already-voted

[bug] #43