consolidated the duplicate definitions of NA values (in parsers & IO)
xref https://github.com/pandas-dev/pandas/pull/16534#issuecomment-305636727

xref #16606 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16589?src=pr&el=h1) Report
> Merging [#16589](https://codecov.io/gh/pandas-dev/pandas/pull/16589?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d298414d2724fed40254350117adcabc4e546e48?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16589/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16589?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16589      +/-   ##
==========================================
- Coverage   90.95%   90.92%   -0.03%     
==========================================
  Files         161      161              
  Lines       49276    49276              
==========================================
- Hits        44817    44805      -12     
- Misses       4459     4471      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.68% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.18% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16589?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16589?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.23% <0%> (-1.82%)` | :arrow_down: |

------

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 @gfyoung look ok to you? Awesome that this works!  My only question is whether we should be importing from `io.common` or from `parsers` (does it really matter, or does the former not work?) i think it should not matter from the execution point of view as parsers import _NA_VALUES from common,  "Explicit is better than implicit."  common makes it explicit, on the other hand not sure what is the role of parsers? is it a gatekeeper? importing from io.common is best will change to io.common @OlegShteynbuk : nitpicking here: I think we generally use `import pandas.io.common as com`.  However, LGTM otherwise. thanks, changed, was not aware of this got the message - 'No test commands were found", local tests were ok.
not sure what happened, is there a way to repeat the tests without pushing changes? ok restarted circle. ping on green. repetition of the same default NA values in io.rst - removed;

in io.rst just after the defaults are listed there is a sentence

  Although a 0-length string
``''`` is not included in the default ``NaN`` values list, it is still treated
as a missing value.

but ``''`` is listed as a default and it is also in common.py  is part of the defaults

should i remove this sentence?
 can u show a screenshot of the built page & also of the doc-string (in ipython) 
![screenshot from 2017-06-08 09 35 45](https://user-images.githubusercontent.com/15165966/26931596-07e77686-4c2f-11e7-95ab-b6669c204669.png)
![screenshot from 2017-06-08 09 41 43](https://user-images.githubusercontent.com/15165966/26931609-1443c736-4c2f-11e7-9b9a-e3a082c4f15e.png)
 have added the label and reference to the label, not the doc-string (in ipython) probably will be the next step lgtm. @TomAugspurger @jorisvandenbossche  thanks @OlegShteynbuk 
have a look at the built docs (prob take a bit to generate) http://pandas-docs.github.io/pandas-docs-travis/

and if anything looks amiss, pls open a PR @jreback built docs are OK, it really took some time to generate.