use message headers instead of different enqueues

this patch removes the functions `sync_enqueue` and `chained_sync_enqueue`;
instead, `enqueue` takes a message header that denotes wheter the message
is synchronous or asynchronous (this patch also prepares libcppa for the
addition of message routing and priorities)
Merge branch 'unstable' of github.com:Neverlord/libcppa into unstable