Allow using different classes for each setting

[Jesus]

This update will allow us to set a custom :class_name for each setting. For example:

has_settings do |s|
  s.key :calendar # Will use the default RailsSettings::SettingObject
  s.key :info, :class_name => 'ProjectSettingObject'
end

This also accidentally solves #57 .

[coveralls]

Coverage remained the same at 100.0% when pulling 29c1ddc on Jesus:master into 659160e on ledermann:master.

[Jesus]

I'll take care of these build errors when I get a chance. Please ignore the PR for now.

[Jesus]

Tests are finally passing, but I think I'll just add some more tests to cover persistence thoroughly.

[Jesus]

Hi @ledermann, first off thanks for your work on this gem, it's been super handy for us.

Please let me know what you think of this PR, if you're willing to support the feature I propose it'd be great.

[ledermann]

Great work, thanks!

[ledermann]

Hm, the changes break my own application using RailsSettings, so I reverted them. Sadly I don't have the time to investigate...

[Jesus]

What versions of Ruby & Rails are you using?