Add secure bootstrapping script for GLBackend.

This implements the issue described in #96
fixed a bug on Linux #92
Fix md5 verification to work with GNU md5sum
Merge branch 'issue/96' of github.com:globaleaks/GLBackend into issue/96

* 'issue/96' of github.com:globaleaks/GLBackend:
  fixed a bug on Linux #92
bootstrap script updated to handle Globleaks main repository.
Merge pull request #92 from globaleaks/issue/96

Add secure bootstrapping script for GLBackend.