Don't carbon copy messages with <no-copy/> hint

Honor the <no-copy/> hint defined in XEP-0334.
Don't store messages with <no-store/> hint

Honor the <no-store/> hint defined in XEP-0334.
Don't log MUC messages with <no-store/> hint

Honor the <no-store/> and <no-permanent-store/> hints defined in
XEP-0334.
Travis CI: Remove unnecessary configure flag

The test suite no longer fails without --enable-transient_supervisors.
XEP-0198: Check whether routed packets are stanzas

Only stanzas are subject to stream management, so when XEP-0198 support
is enabled, we must distinguish them from non-stanza elements.  This
commit adds a send_packet/2 function that can be used in place of
send_stanza/2 or send_element/2 whenever a packet is delivered that
might or might not be a stanza.
Fix bug when joining empty path
Update ejabberd_xmlrpc documentation URL
MUC messages with ~ were not logged (EJAB-1696)
Fix URL markup in the guide
Build proceed/compressed elements in a sane way
Fix formatting
Sanitize copy-pasted get_statustag/1
Fix check_from/2 formatting
If log uses file:write, no need to double escape ~ in messages (EJAB-1696)
Don't use cached passwords if "extauth_cache: 0"

Regarding "extauth_cache", the guide says: "The integer 0 (zero) enables
caching for statistics, but doesn't use that cached information to
authenticate users."  Make sure the cached password isn't used even if
the user is currently logged in with another resource.
Fix proxying of ACK requests for 2xx responses
install epam binary when available
don't stop make install if epam is not compiled