fix error messages and comments
some regex adjustments

IP addresses are now checked for correct charset rather than allowing
everything.
rename !cleartopic to !resettopic

The command resets the topic to the default as configured in uhub.conf.
"clear" wrongly implies that the topic will be emptied.

Also added a plugin description in plugins.conf.
Merge remote-tracking branch 'upstream/master'
Support certificate chains
Fix memleak
Fix struct mixup
mod_topic: check argument for NULL

Better safe than sorry.
Merge pull request #19 from Tilka/master

Bunch of fixes