Implement Akima1DInterpolator
- [x] closes #7588
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

This build failed https://travis-ci.org/pydata/pandas/jobs/121843938 likely because of the `Scipy` version.

`axis` was added to `Akima1DInterpolator` at `0.17.0` as per https://github.com/scipy/scipy/commit/b0395b40c577042436511ce13b9bc067508956db

Is there a minimum version of `Scipy` for `pandas`? What would be a good way to handle lower versions?

Thanks
 you can you call this in a try: except to handle the scipy signature differences. (then if it doesn't work in the excpet, raise again). we don't have a min version. 
 The two test failures seem to be unrelated.
 The two test failures seem to be unrelated.
 pls add a whatsnew as well
 Added whatsnew
 pls squash
 add a small note when to use this in : http://pandas-docs.github.io/pandas-docs-travis/missing_data.html#interpolation (you might want to break up that section a bit and make it into a table)
 add to the doc-string for `.interpolate` with a `versionadded` tag
 Added notes and versionadded tag
 Updated per comments.
 ok, looks good. ping when green (and pls verify that tests are skipped as appropriate)
 @gliptak I just added some more doc comments, looks very good for the rest!
 @jreback Build is green. Tests skipped https://s3.amazonaws.com/archive.travis-ci.org/jobs/122101334/log.txt

```
#268 pandas.tests.test_generic.TestSeries.test_interpolate_akima: no scipy.stats module
#269 pandas.tests.test_generic.TestSeries.test_interpolate_corners: no scipy.stats module
#270 pandas.tests.test_generic.TestSeries.test_interpolate_pchip: no scipy.stats module
```

I also opened https://github.com/pydata/pandas/issues/12856 (consistently fails the same way)
 @gliptak thanks!

pls review the built docs for accuracy (and good looking ness) and issue a follow up if necessary. (prob will take a few hours before they are built)
 @jreback @jorisvandenbossche Thank you for guiding this through
 Docs updated good.
