Otr log command

This commit completes the process of conforming the log policy of weechat-otr with pidgin-otr (v4.0.0) 

See the discussion on the OTR-DEV Mailing-List (here: http://lists.cypherpunks.ca/pipermail/otr-dev/2013-March/thread.html#1655 ) as well as the related previous pull request (here:  https://github.com/mmb/weechat-otr/pull/10 )

This particular commit adds the possibility to start and stop the logging of a given ongoing OTR conversation, regardless of the global logging policy.

Whatever the global logging policy or the last logging status of an OTR conversation, the previous logging configuration of the conversation will be restored as soon as the OTR session is closed. This is to avoid any confusing interference with non-OTR logging settings.

Appropriate warning and informative messages are shown to the user when necessary to make sure no confusion as to the present stare of logging is possible.

Additionnaly, the /otr log command used without a [start|strop] argument will tell the user if the OTR conversation is currently logged or not.


This pull request will stay open for a short period for further bug testing. If the quality of the code seems acceptable, it will then be merged.