Relax ActiveRecord dependency: from 3.0 onwards
Missing ActiveSupport requirement
Rails 3.x compatible

ActiveRecord 4.x ::all method returns relation and replaces former
::scope method.  However `::where(nil)` works across versions.

See: http://stackoverflow.com/q/18198963/304175
Remove unused SaferTokens::Model

Orphaned remnant of some unknown idea.
Use Module::autoload
Constant renamed

DEFAULT_TOKEN_GENERATOR => DEFAULT_CHALLENGE_GENERATOR.
Improve spec: stub proper method
Missing default options spec
Allow defining custom generators with procs
Custom generators receive model as a parameter
Allow using symbols as custom generator option
Validate options passed to Column::new
wip readme