Kill ChangeLog with fire

This is not 1999. We have working version control now.
i18n: Clarify the "Contributors" paragraph in the about dialog for translators

Fixes bug #709003
pixmaps: Fix the installation of effects and transitions icons

This ensures they install into subdirectories of the pixmaps directory,
otherwise Pitivi wouldn't find them when installed system-wide.
Releasing Pitivi 0.91 "Charming Defects"