Add debug option to extension

With this change It will be possible to configure the extension during development to see the console output but it will be hidden by default for end users.

```ruby
activate :php do |o|
  o.debug = true
end
```

**I also updated:**

* How the extension is registered to match the same style as other extensions out there.
* The gemspec so that middleman-core is a dependency instead of a runtime dependency.