implemented suggestions in 919

I have implemented the code but have not yet tested
Update ui.py

removed unneeded comment
updating documentation
Update fill_values docs
Minor fixes in ascii.ui in documentation
Add tests of missing value handling
Update CHANGES.rst to describe API change
Add doc info on fill_values default; Update structure of Using io.ascii
Add example of using fill_value to fill masked entries
Merge pull request #919 from taldcroft/ascii/blank-missing

Interpret blank field values as missing in io.ascii.read