Development model

While I agree that squashing all commits to avoid a huge .git folder makes sense to speed up cloning of mxnet, I believe you should not delete old commits from `dmlc/cub` as it breaks checking out older versions of mxnet that relied on those.

Checking out old versions of mxnet can be important when debugging changes introduced between two revisions (git bisect). Another usecase is when having some code that was not touched for a while and used to work but does not work with current mxnet master ;)

@mli, what do you think? Of course you can't change this retrospectively, but perhaps you agree that it makes sense not to change the history of this repo in the future?