DEVELOPER_README: remove dependence on atool/apack

The common command `zip` (Info-ZIP) has a fairly
simple and standard way to create an archive and
atool uses it.
OS X: build system: make putting Lasercake in a .app bundle optional
make dev-build.py work for more platforms

Windows had been broken in that running tests called
`Lasercake` rather than `Lasercake.exe`.

OSX when creating an OS X bundle (cmake -DOSX_BUNDLE)
had been broken too.  Now it works.  (It's slower to create a
bundle than not, so dev builds likely don't want to build bundles,
but it's good to have it work.)