Loghandlers (#817)

I have a basic implementation of logging handler plugins - triggered by a signal. Though, I'd say that signals were not needed at all for this plugin to be added. Oh well - they're probably useful for other things.

Logging throughout Nikola is quite incomplete. ~~Some error messages such as those from the rest compiler are not captured by Logbook (yet) - so the resulting error message is frustratingly vague.~~ 

~~For example, if you make a syntax error in a rST document you will get the message `Docutils reports warnings on posts/test.rst` rather than something useful like `Docutils reports warnings on posts/test.rst - <string>:3: (WARNING/2) Title underline too short.`~~

~~However, if merged, those more l33t than myself can work out how to get the error messages on stderr to be captured by Logbook.~~