Theme support

Fixes issue #3

### So much themes, so fun, so classy !
1. All themes are now available using `wpackagist-theme/THEME`
  * In my tests, I saw that some old themes are not hosted anymore, their download page redirect to the themes index page, but they are still in the SVN. This causes composer to download the HTML of the index page as if it was a ZIP file and crash weirdly. However, I don’t think this is a big issue as you should probably not be trying to download a deactivated theme.
2. Plugins are now available using `wpackagist-plugin/PLUGIN`
  * The old `wpackagist/PLUGIN` is still working properly but is now deprecated.
3. Documentation in `index.html` was updated accordingly.
  * N.B. The plugins used in example were changed because the names were too long for the box.

### Heavy internal redesign
1. Adds a package abstraction layer:
  * `AbstractPackage`
    * `Plugin`
    * `Theme`
2. Database changes
  * Name changed from `data/plugins.sqlite` to `data/packages.sqlite` to better reflect its new content.
  * New column: `class_name` which the fully qualified class name. This is to use `PDO::FETCH_CLASSTYPE`
  * The primary key is now `(class_name, name)`
3. It is no longer possible to pass the base SVN url in parameter
4. All the JSON generation logic is moved from `BuildCommand` to `AbstractPackage`