BUG: int dtype for get_dummies
closes #8725

Changes `get_dummies` return columns with `uint8` dtypes instead of coercing to floats if they were alongside other float columns.

pls give a test on windows, as int dtype comparisons are wonky.
 actually this may not work w/o #667 

@sinhrks would know more
 Right I should have put a WIP in the title, sorry. That seems like a bit of a rabbit hole, but I'll give it a bit more time this week if I can.

> On Jul 27, 2016, at 05:42, Jeff Reback notifications@github.com wrote:
> 
> actually this may not work w/o #667
> 
> @sinhrks would know more
> 
> —
> You are receiving this because you authored the thread.
> Reply to this email directly, view it on GitHub, or mute the thread.
 I'm just working on #667, hopefully can send something this week... :)
 Awesome, thanks :)
 this has to be after #13849 
 @TomAugspurger I merged the sparse PR of @sinhrks, so if you have the time, you can update this now
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13796?src=pr) is 85.27% (diff: 100%)

> Merging [#13796](https://codecov.io/gh/pydata/pandas/pull/13796?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13796   diff @@
==========================================
  Files           139        139          
  Lines         50553      50554     +1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43110      43111     +1   
  Misses         7443       7443          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [58199c5...cace0f7](https://codecov.io/gh/pydata/pandas/compare/58199c5da263491a26fa6cb26d2ff9d38c4b5dac...cace0f7c550dea5197b7aa5eb0cbd49b357ee8b7?src=pr)
 Green now. Lots of (small) changes to the tests, so a second pair of eyes would be grateful.
 Looks good to me
 thanks!
 this makes this very memory efficient now! (and of course sparse is even better)
