Cover mistakes found while implementing
https://github.com/ziontab/lua-nginx-tarantool

Regression introduced by 1376aad9

Add a Gitter chat badge to README.md

Misc cleanups & renames.

Fix a bug in sio_writev_all with possible
going beyond allocated memory.

Log file/line in system errors.

If replication handshake failed, panic, don't try to
reconnect and handshake again.

This will simplify client library construction cause it will allow to use
streaming MsgPack decoding.
If reply has variable part count, then request decoder which uses streaming
decoder, need to wait next part to analize: is it belongs to this reply
(body map) or start of next reply (ie length integer)? So that, if request
(ping) were single request, request decoder will stuck.

Unit test coio_stat.

Cleanup unit tests a bit: merge unit.h and test.h, which
would duplicate each other.

Fix a bug in trigger_atexit.test, which would not clean up after
itself.

Actually print an error message and class name in the log.