Implement inertial drag

Please see the example: http://bl.ocks.org/pjanik/5872514
Recently I needed such feature and implemented it in my application, but then I came up with the idea that it can be easily generalized. Inertial drag is quite popular nowadays.

I'm creating pull request so you can easily review the code, but of course I'm open for discussion. 
Do you find it useful?

Note that I'm not very familiar with d3 codebase, so I'm happy to fix implementation details, style, add some tests etc. in case of need.