Add Gearman::Server class

`Gearman::Server` is a uninitialized constant, maybe forget to require it ?
```
2.1.5 :001 > require 'gearman'
 => true 
2.1.5 :002 > Gearman::Server
NameError: uninitialized constant Gearman::Server
        from (irb):2
        from /home/ztz/.rvm/rubies/ruby-2.1.5/bin/irb:11:in `<main>'
```

Add `require 'gearman/server'`