Modernify edd_download_meta_box_save()

This PR does a few things:
* Cleans up coding standards (whitespace, braces, Yoda)
* Removes unused $edd_options global
* Adds second parameter to the save_post hook.  This allows for less reliance on global $post, which can be too easily stomped, and should be backwards compatible.
* Adds 'edd_save_download' action with $post_id and $post passed to it.  Obviously, plugins can also hook into save_post, but it seems more logical to have a unique action to hook into, so third parties don't have to do the same sanity checks that fill the first 15 lines of the function.