Custom protocol support

[eile]

In relation to @julitopower work:

• Add a protocol magic identifier as first word on a connection
• Safe protocol with connection
• Implement current code path for co protocol
• Add raw protocol
• Allow protocol -> boost::function( ConnectionPtr ) registration for custom protocols which will be called when data arrives on a non-co connection

[eile]

Use case: other protocols can be dispatched using co::LocalNode.