ENH: Allow usecols to accept callable (GH14154)
- [ X] closes #14154 
- [ X] tests added / passed
- [ X] passes `git diff upstream/master | flake8 --diff`
- [X ] whatsnew entry
  <img width="1515" alt="asv_bench" src="https://cloud.githubusercontent.com/assets/609873/18575075/942a9240-7ba1-11e6-9dca-bab8b9987f31.png">

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14234?src=pr) is 85.28% (diff: 100%)
> Merging [#14234](https://codecov.io/gh/pandas-dev/pandas/pull/14234?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14234   diff @@
==========================================
  Files           144        144          
  Lines         50953      50963    +10   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43452      43462    +10   
  Misses         7501       7501          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [4378f82...9b0d112](https://codecov.io/gh/pandas-dev/pandas/compare/4378f82967f59097055eef17ede50aa515525551...9b0d112b7c18c4f31b66a283cfde383f9fec21cc?src=pr) can you rebase, move whatsnew notes to 0.20.0, and add an example as indicated Requested changes should have been made - let me know if there's anything else you would like to tweak for this enhancement @WillAyd thanks. couple more pretty minor comments. otherwise lgtm. All of your requested changes should be accounted for.  Let me know if anything else comes to mind. I noticed that the Travis-CI build failed for OS X.  I didn't see anything useful in the log and checking their website it appears that the failure may have stemmed from issues on their end and not necessarily from the code.  Do you have a preferred method to restart that build?  Should I close / re-open the PR or do you have another way you'd like to restart that test?

<img width="868" alt="screen shot 2016-11-29 at 7 01 45 pm" src="https://cloud.githubusercontent.com/assets/609873/20734278/66c770c0-b666-11e6-9525-e87de7899461.png">
 I restarted the job. @WillAyd just a couple of minor comments. ping when green. @jreback pushed some edits to cover your latest comments.  Let me know if there's anything else you'd like on this lgtm. ping on green. @WillAyd Thanks!