[IZPACK-1442] - Ensure not to display splash screen on console and automatic installations

This is to address http://jira.codehaus.org/browse/IZPACK-1142

1. Splash screen information is no longer added to the MANIFEST.MF so that we gain control on when the splash screen should be displayed.
2. Specification to add a splash screen has been updated, and this will be updated in the online documentation. See below for a sample of what is needed to define a splash screen.
```
<guiprefs width="800" height="600" resizable="no">
    <!-- Show the splashScreen for a minimum of 1000 milliseconds. -->
    <modifier key="useSplashScreen" value="1000"/>
</guiprefs>
<resources>
    <!--Define the splash screen image as a reading just like a heading image -->
    <res id="Splash.image" src="images/splash.png"/>
</resources>
```
3. Tests updated to check for the splash screen resources "Splash.image" if it has been defined properly rather than in MANIFEST.MF

Note: This will cause older installers not to display a splash screen until they update the the specifications above.