Fix for IZPACK-1009; update javadocs to have fewer missing piece of information.
RulesEngineImpl changed so that operands are not blindly added to the list of
operands for AndCondition / OrCondition / XorCondition. Instead, each condition
(and sub-condition) is checked for existence before being added; if any operand
in any expression doesn't exist, the whole expression will evaluate to false.
fix the complex expressions also
IZPACK-1109: align the warning messages to use the same language
Merge pull request #246 from Shrafe/IZPACK-1109

Izpack 1109
Merge pull request #247 from mtjandra/IZPACK-1442

[IZPACK-1442] - Ensure not to display splash screen on console and automatic installations
Only try to load splash screen resource if defined in guiprefs.