require old dmd version as separate command line argument

- using git describe doesn't work reliably in the following cases

  - uses pre-releases versions instead of the last release
  - skips tags that do not differ from HEAD (in the installer repo)
  - fails to determine ancestor when the release branch
    isn't merged back into master
only build installers for tagged versions

- still allow branch builds without installers
fix issue when building only a single box
build installers for other OSes

- updated debian version (required by linux installer scripts)

- build OSX dmg

- use version without 'v' prefix for zip file names, e.g. dmd.2.066.linux.zip

fixup installer build
remove skipping a box when it was already

- rather confusing during development and pointless
  during building
put all output files into a build folder
Merge pull request #113 from MartinNowak/improveBuildTool

improve build script