improve the interface to better support unix socket.

delete client.host
delete client.port
add client.address
add client.connectionOptions

add tests.UNIX_SOCKET

update all error message to use client.address
update retry connection
updated readme of the createClient() part
fix the example in createClient
Fix tests for older versions of Redis.
Merge branch 'jackhftang-master'
Update changelog
0.12.0