Suppress warnings with bundle exec.

Hi,

Could you consider to merge this change?
When I executed clockwork with bundle exec, it resulted in some warnings like the following

```
$ bundle exec clockwork clockwork.rb 
/usr/local/rbenv/versions/1.8.7-p371/lib/ruby/gems/1.8/gems/clockwork-0.5.1/lib/clockwork.rb:10: warning: already initialized constant NOT_SPECIFIED
/usr/local/rbenv/versions/1.8.7-p371/lib/ruby/gems/1.8/gems/clockwork-0.5.1/lib/clockwork.rb:11: warning: already initialized constant WDAYS
…
```

It can be solved by modifying required argument in bin/clockwork.

```bin/clockwork.
- require File.expand_path('../../lib/clockwork', __FILE__)
+ require 'clockwork'
```

I think that this change is appropriate with the following instruction. 
http://guides.rubygems.org/make-your-own-gem/#adding-an-executable