Update app icon and description
Update some debian specific files

* Update postrm to use aegis-exec to remove GagBook config files as user
instead of using aegis manifest for executing the whole script as user
* aegis manifest now auto generated (result is empty manifest)
* Update changelog file to be more standard
* Update copyright file to follow the specification from
http://www.debian.org/doc/packaging-manuals/copyright-format/1.0/ (this
specification is optional)