Ntp synchroniziation

This should theoretically allow clients to sync the output of audio between different sinks.

This is a WIP branch that i'm playing with to allow XBMC sync airtunes sinks. It's not in final form and i'll likey rebase this a few more times. But comments are very welcome.

Note.. This breaks API/ABI but that seem impossible to get around with current method of allocating the callbacks interface structure.

There also seem to be lacking a library version number in headers to allow users to atleast IFDEF around the API breakage.