Warn about unquoted node names

According to the Language Reference:

"A node statement’s name must be one of the following:
* A quoted string
* The bare word default
* A regular expression"

http://docs.puppetlabs.com/puppet/3/reference/lang_node_definitions.html

But puppet-lint doesn't complain if your node names are unquoted. For example:

```
node server1 {
  ...
}
```

while still accepted by Puppet, does not conform to the Language Reference rules. I'd like to see puppet-lint warn you, perhaps by saying:

WARNING: unquoted node name on line 1