Update cura.desktop

Added path to cura.desktop in order to fix slicing errors on Ubuntu.
Update requirements.txt

Added missing prerequisites/dependencies for Ubuntu. 
Update README.md

Added build instructions for Ubuntu.
Merge pull request #377 from dylanwinn/master

Fixed errors in Debian packaging.
Remove the cx-freeze requirement.
Merge branch 'master' into SteamEngine
Add steamengine support configuration.