Add support for icon_hash

A rewrite of #425 which uses the xattr attribute com.googlecode.munki.sha256 to store the current hash. 

Check for the addition of an icon_hash key in a package plist. If the key is present and matches a hash of the corresponding icon that has already been downloaded, no attempt will be made to download the icon/verify the icon with the server. If the icon_hash is not present or the hash does not match the standard procedure for checking icons will be followed.