add the ability of registerWeak, which could help reduce memory leak

[ice871117]

we have been using this registerWeak function for a while and it runs steady, also reduce the chance to cause memory leak on android. sometimes it is really hard to find a proper place to call unregister. Empty reference will be removed when new registration happens

[GregoryConrad]

@greenrobot-team Any update on this? This would be an amazing feature to have and the idea was brought up all the way back in January 2014: #57

[greenrobot-team]

No update. But as a general hint, you should not expect such fundamental (read potentially breaking) changes making it into the library.