Added possibility to use alternate download locations (sparkle:os attribute)

Added possibility to use alternate download locations in different operating systems
by specifying sparkle:os attribute in appcast.xml file. 

Appcast.xml can contain one <item> for Mac OS X
and one <item> with <enclosure sparkle:os="windows" ...> for Windows.

If no sparkle:os="windows" attribute found, the first <item> is used as usual.