User super: true to access overriden accessors

[shioyama]

Mobility::Translates aliases any original setter and getter methods to e.g. title_before_mobility so that they can still be accessed, but this is pretty ugly.

I did two things in this PR:

• refactored out this pattern in the only place it is used internally (in some Sequel backends) so it's not needed for Mobility itself anymore
• replaced the _before_mobility alias pattern with a super option, so you can call the original method (if one was defined) with article.title(super: true)