Installer: Rename logo.bmp to logo-small.bmp to match Inno Setup's naming

The logo is used as a "WizardSmallImageFile".
Reduce logo-small.bmp file size by using 24-bit instead of 32-bit colors

The image has no alpha so there is no point in storing it.
Installer: Add a (large) WizardImageFile

The bitmap was rendered from the SVG using ImageMagick, the text was added
using Gimp (Calibri Bold, color same as scissors).
Merge pull request #208 from sschuberth/master

Add a large wizard image to the first installer page