PeriodIndex count & size fix
closes #12774 
closes #12868 
closes #12770 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Initial attempt at fixing some of the more urgent issues

@jreback green
 @MaximilianR thanks!

I used your defs of `downsample_methods` and such and found a buggie, similar to what you just fixed, e.g. #12886 
 @jreback , @MaximilianR : I'm not sure what happened when this PR was committed to `master`, but it broke Travis AFAICT (even though the tests here in this PR are passing).  Can replicate the failures locally FYI (i.e. no failure on 2.x, but failure on 3.x).  Does it have to do with #12886?
 I was testing some more things which happened to fail on py3. Reverted for now, the issue is #12866 which is broken.
 @jreback : I presume you mean #12886?  Yeah, I don't quite understand the exceptions caught either that you get with 2.x...confusing... :confused: At least the tests for `test_resample.py` are all passing again locally :smile: 
 yeah somethings wrong not sure what
 Just saw this. Are there any more details on the breakage?
 no this is all fixed though see the linked issue
 OK, thanks
