Fixed runtime error due to not including YAML lib

I tried to restore from a save state that I had previously made, and got an error:

```
# ./smbexec.rb -S log/smbexec-2014-11-17/state/smbexec_state_14-59.yml 
./smbexec.rb:62:in `block (2 levels) in <main>': uninitialized constant YAML (NameError)
	from /usr/lib/ruby/1.9.1/optparse.rb:1391:in `call'
	from /usr/lib/ruby/1.9.1/optparse.rb:1391:in `block in parse_in_order'
	from /usr/lib/ruby/1.9.1/optparse.rb:1347:in `catch'
	from /usr/lib/ruby/1.9.1/optparse.rb:1347:in `parse_in_order'
	from /usr/lib/ruby/1.9.1/optparse.rb:1341:in `order!'
	from /usr/lib/ruby/1.9.1/optparse.rb:1432:in `permute!'
	from /usr/lib/ruby/1.9.1/optparse.rb:1453:in `parse!'
	from ./smbexec.rb:78:in `block in <main>'
	from /usr/lib/ruby/1.9.1/optparse.rb:882:in `initialize'
	from ./smbexec.rb:7:in `new'
	from ./smbexec.rb:7:in `<main>'
```

This patch should add the YAML library back in, allowing it to parse the state again.