[IZPACK-1129]

Proposed fix for http://jira.codehaus.org/browse/IZPACK-1129 (Unattended install still prompts user for decision on overriding files)

This fix makes the unpacker use the default choice  instead of asking the user for a decision during an automated install. The default choice depends on the override attribute:
Overwrite by default if override="askTrue"
Do not overwrite by default if override="askFalse"

Note: I added a static method to Installer class used to determine the type of installation being done (GUI, console, or automated). Please feel free to suggest any better ways of getting this information during runtime.