Make -NaN and -nan default NA values
closes #5952  

Probably `test_default_na_values` should be brought into alignment?
 @Bklyn yep on `test_default_na_values`...in theory that should fail with your change (until its fixed)....does it?
 Sorry for the noob-ishness, but what is the suggested way to run the tests against not-yet-installed version of pandas (e.g. from my working copy)?  I'm getting import errors if I run the test script by hand unless I "setup.py install" first.

At any rate, the tests do _not_ fail if I don't change them, presumably because they're not trying the new -nan values.   Will fix the code and update the PR with the tests for completeness.
 @Bklyn make this part of the same test (which SHOULD fail if the _NA_VALUES are not updated)

e.g.

`assert_array_equal(_NA_VALUES,pandas.io.parsers._NA_VALUES)`

This way the test have to be updated as well
 @Bklyn 

`python setup.py build`

then 

`nosetests pandas/io/tests/test_parser.py`
 Is there anything left for me to do on this PR?
 pls add a release notes entry (put in 0.13.1 API), 

rebase and squash to 1 commit: https://github.com/pydata/pandas/wiki/Using-Git
 OK, I think thats got it.
 @y-p @jtratner ok by you?
 fine by me.
 thanks  @Bklyn 

nice little PR, and you were very responsive!!
 Thanks for the hand-holding and the feedback.  You guys (and perhaps gals?) do a great job with pandas and your attention to code and documentation quality is impressive.
 welcome! feel free to tackle any open issues!
 Yeah, we need a gal commiter.

Well, a ninja that happens to be a gal.
