Reload only if Application#reload_dependencies? returns true

Rails 3.2 reloads classes only when files actually change,
so if Application defines the method reload_dependencies?
(only defined in rails 3.2 so far) check if this method
returns true before calling cleanup! and prepare!. This way,
we don't reload classes unnecessary.
Merge pull request #9 from arturopie/rails-3.2-conditional-reload

Reload only if Application#reload_dependencies? returns true