Use updated syntax for WP CLI.

Looks like the syntax required for registering WP CLI commands has changed at some point and these commands weren't working correctly. 

* I have updated the backup command to use the docblock syntax. 
backup
* The `backup` command is now a subcommand of `backupwordpress` - this allows for new functionality in the future.

Now you just need to do `wp backupwordpress backup`