Remove Globalize support

We'll be adding this to a separate gem for FriendlyId 5.
Provide finders in scope; don't override base finders
Implement candidates, remove sequencing.
Update required versions of everything
Remove unused model_class attribute from SlugGenerator
Remove ActiveRecord 3.1 support
Made history work with new scope-based finders
Clean up SQL
Make slug deletion consider scopes
Improve method name
Add scopes file
Rename variable for clarity
Stop Bundler's complaining
Remove outdated info
Simplify module loading in configuration
Use class eval rather than instance eval
Don't subclass SlugGenerate unnecessarily
Change default sequence separator to "-"
Document Rake tasks
Update README