ENH: Allow abs to work with PandasObjects
Add `__abs__` method so that you can use the toplevel `abs()` with
PandasObjects. I note there aren't that many existing test cases for
abs. At some point we might want to think about that.

Also related #4819 

cc @thisch

if no objections, will merge this soon.  Entirety of the change is adding this:

```
def __abs__(self):
    return self.abs()
```
 this is fine....just an oversight, I think < 0.13, `abs(a_series)` worked because numpy defined **abs**
 Yep numpy does define abs. Looks good. 
 thanks for pointing this out @thisch - now all pandas objects support abs and your original code should now work.
 what the heck? I added release notes to this and pushed them to Github before merging, but they didn't end up in this PR. weird. Worth committing separately @jreback ?
 ha....just was noticing that...no...you can throw them in next time you merge something
 no test for poor-old `Series` :cry:?
 It gets tested in test_panel (weirdly). If we want to add abs tests, we should do it to test the abs method...this is basically just adding some sugar :)
 ah ok yep i c now
