Logbook panel

![ 2013-02-19 6 29 02](https://f.cloud.github.com/assets/249795/170655/e90372ee-7a76-11e2-88d5-46e6e0face4f.png)

New panel for [Logbook](http://logbook.pocoo.org/). Enabled by default, but has a dependency on Logbook package so in the environment without it, will disabled automatically.

Most of code copied from `flask.ext.debugtoolbar.panels.logger`.