Dropdown auto position.

Created by: sathyamoorthi

I'm sure this should have been discussed. But i'm only seeing this OLD thread (https://github.com/twbs/bootstrap/issues/1411) regarding this issue. I did small piece of code to achieve this. If you wish i can give PR with that. Else, someone point me out the real reason why this should not be added into the core?

-Thanks.