Add the possibility to configure logging policies

Following the discussion on the OTR-DEV Mailing-List on logging policies ( http://lists.cypherpunks.ca/pipermail/otr-dev/2013-March/thread.html#1655 ) I have decided to conform weechat-otr to the logging policy of pidgin-otr (v4.0.0)

With this commit the behaviour will be:

- All OTR sessions are NOT logged in any way by DEFAULT
- The user can override this policy to allow systematic logging of OTR sessions
- Whenever OTR logging is activated, the user is prompted with a warning and is told how to revert this dangerous policy if he wants to.

A future commit will complete this conformisation further.


This pull request will be kept open a few days to allow for comments and suggestions, and will then be merged.