Define which args are used to determine uniqueness.

Addresses a use case where only certain arguments should be used
in determining if the job is unique or not. The default to hash
all arguments is not always preferred.
Update readme regarding unique_args method
Use klass.constantize

- eval is evil
- we realized that Sidekiq adds core extensions to String
  if active_support is not loaded
Merge pull request #12 from wanelo/master

Allow worker to specify which arguments to include in uniquing hash