instantiate connection object before each test
test for build_stanza
update gem dependencies
implement build_stanza
let req_* methods return single hash to build stanzas
merge connection into client

Move the following connection functions into client:
  log
  log_error
  build_stanza
  params_to_xml

The `request` method will be reimplemented later.
remove connect and close methods
use Libertree::Client rather than ::Connection in tests
rename test file
add TODO markers
use fetch instead of ||
open UNIX domain socket to talk to the XMPP relay
The command is part of the params hash
let req_chat build a complete XMPP stanza
rename server_ip to domain

We need to know the domain of the target XMPP server (actually the
domain of the XMPP server component, i.e. the remote Libertree backend),
not the raw IP address.
request method: pushes stanzas through XMPP relay
connect to socket and catch connection errors
use `request` with target and stanza generators