tag synonyms

model, management command, and test code

Hello Evgeny 

I added tag synonyms to askbot. 
[who-pre-sets-the-tags-admin-or-users](http://askbot.org/en/question/2773/who-pre-sets-the-tags-admin-or-users/) 
(pjunks is me)

This is my first attempt to contribute to a project using git/github.
Please let me know if I missed something.

Thanks for a great app by the way.