Fix #380 - auto manage wpcom user blogs

Automatically manage WordPress.com user's blogs
* WordPressDB schema modified to add a "hidden" field in the "accounts" table
* After sign in, all blogs are automatically added
* Add a new settings "Show/Hide blogs"
* In settings "Add Blog" replaced by "Add self-hosted blog"
* Remove the "Remove Account" button from WordPress.com blog settings
* Refactoring