Added core support

Fixes #4

This package does NOT handle the installation or any configuration of Wordpress Core, but it does allow to download it using `wpackagin/wordpress`.

It currently fetches a list of tags from the SVN repo, adding `trunk`.

For each tags, the `dist` is a tarball in the form of `http://wordpress.org/wordpress-VERSION.tar.gz` and the `source` is the SVN repo.

For `trunk`, the tarball is `https://github.com/WordPress/WordPress/archive/master.tar.gz` and the `source` is the SVN repo.

All this is probably not desirable and is up for debate, but at least it is working.

**UPDATE**: Do not merge, I found bugs, I’ll work more on it later.