version 2.1.0

Merge pull request Yelp#19 from Yelp/basher_cocoadocs_cleanup

Documentation cleanup

Merge pull request Yelp#10 from Yelp/YLTableViewCellEstimatedRowHeight

YLTableView 2.0 - YLTableViewCell as protocol

Merge pull request Yelp#6 from mglidden/mglidden-swift

Made models be ids for better Swift compatibility

Merge pull request Yelp#5 from ytok/modernization

Adding nullability annotations, minor modernization updates

Change back to https source now that the repo is public

Use ssh repo instead to play nicely with GitHub 2FA