[IZPACK-1141] Automated installation prompts user for missing values

This PR addresses http://jira.codehaus.org/browse/IZPACK-1141.

The AutomatedInstaller will now prompt the user to enter any missing values for userInputPanel entries found in the automated xml. 

The XML entry must have no 'value' attribute to trigger the user prompt.
<entry key="some.variable" />

A couple of notes:
- This PR only adds this functionality for UserInputPanel entries. If others want, I could look into implementing the same system for other panels that require user input.
- The prompt displayed to the user is generic: it does no validation or password masking for example.