Add the ability to include Vanilla dependency.

I made use of Maven's capacity to handle preprocessor directives in the XML files. If the flag is `false` (the default), no trace of the Vanilla-specific code will be included. If it is `true`, then Vanilla is included as a Maven dependency, and is added to the `depends` section in the `properties.yml` file.

Atheriel <atheriel@gmail.com>