watch function return new thread object for unwatch

`watch` function return new thread object so that thread can be killed by code
code like that:
```ruby
pin_11_thread = watch :pin => 11 do
    puts "I am just be watched"
  end

## unwatch pin 11
pin_11_thread.kill
```

use `kill` method seems not brilliant but I can find another way to write a function like 
```ruby
unwatch :pin=> 11 ......
```

is this way ok?