Corrects Ubuntu/Debian and Manual installation instructions

The package list contained various invalid package names (which was also the topic of several forum threads already, see e.g. http://forum.owncloud.org/viewtopic.php?f=3&t=3725 or http://forum.owncloud.org/viewtopic.php?f=3&t=2345 .
This should correct the list and the commands, the commands were tested on Ubuntu 12.04 LTS server.

Furthermore, for an installation via the distribution's package management system, Debian/Ubuntu were missing; i added these with a link to the package list.