BUG: value_count(normalize=True, dropna=True) counting missing in denominator, #12558 
Closes #12558 

#12566 fixes this too, will need to fix groupby. Lemme know if you wanna work on this and I'll close mine. 
 @kawochen Oops! Sorry, did not mean to double up. If you have this covered I can close -- not sure what you mean about groupby. 
 `groupby` has a version of `value_counts`.  I fixed it for `numpy` > 1.8 and have an idea for older versions.  Lemme play around with it tonight (EST).  If it doesn't work I'll close it and let you have fun :)
 @kawochen Sounds good! Just let me know. 
