Don't let proguard shrink module constructors

Since the injector construct module by reflection, Proguard may
remove these if they're not called explicitely anywhere else, which
would result in uninstantiable modules.
Make sure module constructors don't get shrunk by Proguard.
Merge pull request #9 from guillaume-mixi/master

リリース版でインジェクションが失敗する問題の修正