Updated background to 72 ppi

For some reason that makes it look proper in the dmg installer.
icns file is now added to the .app

icns file is now added to the .app as well as the plist, making Mudlets
icon be properly used in the resulting .dmg.
Added installation instructions file

Mostly describing how to build the nice .dmg
Removed warnings on OSX

The paths never get defined anyway and always errored.