Allow counting on object columns with nan

These can show up from pandas string columns
Test that we can count strings with nans
Count should return an integer type
Add test for dtypes
We have to test this in bcolz
Fixup datetime counting
Test that we are doing chunk nunique and count on datetimes properly
Cheap check for nan with datetime64s of any frequency
Dshape should be option for date here
By definition we are an integer no need to specify the dtype
Merge pull request #902 from cpcloud/np-agg-bug

fix to allow count of object columns and count,nunique of datetimes of any frequency with possible nats