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.
Don't miss incoming presence updates
Merge pull request #212 from weiss/fix-presence-updates

Don't miss incoming presence updates
Fix bug when joining empty path
Update ejabberd_xmlrpc documentation URL
Merge pull request #213 from weiss/fix-xmlrcp-doc-url

Update ejabberd_xmlrpc documentation URL in the guide
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.
Omit XML namespace declaration for <iq/> stanzas

Only the child elements of <iq/> stanzas are qualified by the namespaces
in question, not the <iq/> stanzas themselves.

This change just clarifies the code.  It doesn't alter the behaviour, as
those <iq/> stanzas are handed over to jlib:iq_to_xml/1, and that
function ignores the 'xmlns' attribute anyway.
Merge pull request #207 from weiss/xep-0334

Honor XEP-0334: Message Processing Hints