[Izpack 1138] - Omitting passwords (or other sensitive info) from automated xml files.

This pull request is for the following issue, https://jira.codehaus.org/browse/IZPACK-1138.

I've implemented it in such a way that if the spec of a UserInputPanel field has the omitFromAuto='true', the value will be omitted from the auto-install.xml

For example this, 
field type="password" variable="admin.user.password" tooltip="admin.user.tooltip" 
     spec omitFromAuto="true" 
field

would appear as follows in the auto-install.xml, entry key="admin.user.password", there should be no value for the xml element