Zero counts in Series.value_counts for categoricals
`Series.value_counts()` also shows categories with count 0.

Thought this would be a bug but according to [doc](http://pandas.pydata.org/pandas-docs/version/0.15.0/categorical.html#operations) it is intentional.

This makes the output of value_counts inconsistent when switching between category and non-category dtype. Apart from that it blows up the value_counts output for series with many categories.

I would prefer to hide counts (i.e. zero) for non-occuring categories by default and rather consider a parameter `dropzero=True` similar to `dropna` (see also #5569).
