New features

Added some new features:
Added counter_messages in JSON output for sequence keys
Added type_of_message which can be used for processing in the backend
Added ppid, variable that holds the hostname and PID of ExaBGP. Can be used when you want to determine the host that served the ExaBGP instance and what the PID of the process was.
Reason of "down" message wasn't passed to the backend, fixed this.

Note:
This pull request is also compatible with the text encoder (so both text and JSON encoder).