Update build.xml to support windows OS

 - Remove instances of `rm -rf` to use phing's dir delete
 - Add `bin.which` property which uses `where` for Windows and `which` for Unix/Linux
 - Update instances of `app/console` to use `php app/console`
 - Update instances of `composer.phar` to use `php composer.phar`

Note that you must run the terminal "As an Administrator" in order to run some of the phing commands.