Clockwork module define in manager class_variable move to instance_variable.

Hello. 
I made ​​changes to move to a singleton variable manager, which is defined as a class variable.

Please try to run the this code in a clockwork stable to try.

```ruby
# /tmp/test_clockwork.rb
require 'clockwork'
include Clockwork

handler do |job|
  puts "Running #{job}"
end

every(10.seconds, 'frequent.job')
every(3.minutes, 'less.frequent.job')
every(1.hour, 'hourly.job')

every(1.day, 'midnight.job', :at => '00:00')

class TestClass
  @@manager = Object.new
end
```
When you run the `clockwork /tmp/test_clockwork.rb`, stack level too deep error will occur...

That this error, class variable from being shared object hierarchy of all the following reasons.
So I have been moved `@@manager` to the class instance variable(singleton variable).
handler method and run method is include by singleton instance and instance in `Clockwork.included`.
should be able to share manager from the object of all this.

My environment success all tests.(Mac OS X 10.8.2, ruby-2.0.0p0)
and, I was able to run successfully and the test of running the command line.

Please confirm.
Thank you.