Update to 0.56

Changelog:
=== 0.56 - Mon  2 Apr 2012 ===
* Merged support for Git and Mercurial version control backends -
  Damien Accorsi & John Drinkwater
* Merged plugin for "ditaa" diagrams - YPWang
* Merged patch for different configuration profiles, allowing per
  notebook configuration of plugins, font etc. - Mariano Draghi
* Added drag & drop support for the Attachment Browser plugin
* Made sidepane and tagcloud remember state
* Fixed critical bug for opening email adresses without "mailto:" prefix
* Fixed bug where context menu for page index applied to the current page
  instead of the selected page
* Added a Serbian translation

=== 0.55 - Tue 28 Feb 2012 ===
* Numbered lists are now supported
* The index now has "natural" sorting, so "9" goes before "10"
* Added new plugin to show a Table Of Contents per page, and allows modifying the outline
* Added Markdown (with pandoc extensions) as an export format
* New context menu item "move text" for refactoring text to a new page
* Tasklist now supports a "next:" keyword to indicate dependencies,
  and it can hide tasks that are not yet actionable
* Made zim taskbar icons and trayicon overloadable in theme - Andrei
* Fixed behavior of Recent Pages pathbar in cases where part of the history is dropped
* Fixed behavior of the Search dialog, it no longer hangs and also allows cancelling the search
* Fixed bug where replacing a word (e.g spell correction) could drop formatting
* Fixed behavior of case-sensitive rename on a case-insensitive file system (windows)

(ryoon)