add ability to set custom fonts for a UIController

Default fonts are still loaded as previously, with the addition of a default "icon" font. Added `setFont` method to allow customization of fonts, e.g.:

    mParams = UIController::create();
    mParams->setFont( "icon", Font( loadResource( RES_GLYPHICONS_REGULAR ), 22 ) );

Also added an exception when an unknown font style is specified for either getFont or setFont.