Use importlib for python 3.3 to load plugins

(also probably fix the I/O operation on closed file issue when using
imp)
Use ''.join(str) instead of str += …

Should improve speed on xhtml parsing, but be noticeable only
with *heavy* (e.g. 1000chars with the rainbow plugin) xhtml stanzas.
Actually use the deprecated method for versions INFERIOR to 3.2
Typo
Use ISO 8601 date format in the day_change plugin
Take into account the nick given by the MUC when joining a room.

This way, if the MUC service decides to give us an other nick than the one
we asked it works properly.
Lets plugins change the presence BEFORE joining a muc, with a new plugin event
Document the new event
Little fix a doc
Add a new random_nick plugin
Actually have a random nick ONLY when our nick is RANDOM
Add a new replace_word plugin.
Make Alt+backspace work on my freebsd
Fix a typo "on on an unknown platform"
Fix #2286

Also fix the copy of the default config if -f is used
Fix #2284 (don’t send always send an inactive chatstate with /w)
Add a nice marketing plugin
dlm
The name is poezio, not Poezio
Add underlined to the doc