Spec jrpc2.0

jsonrpc-c is more strict with the specification jsonrpc 2.0 in this branch.

Major changement : 
- Need jsonrpc version
- Better management of id.
- Request is notification when missing id.

What missing about the spec : 
- Management of batch
- No reserved name like "rpc."
- Don't check if id is not a float

This branch include abstract_server.