address #910 (fill_values in astropy.io.ascii.write)

This implementation solves the problem discussed in #910.
I suspect that it costs some performance for large tables (I did
not actually perform tests for that), because I replaced an
iterator with an explicet list.
However, in this way most of the fill_values logic can be reused
from the implemention of fill_values on input.
PEP8 fixes in astropy/tests
zip -> izip
Merge pull request #910 from hamogu/fix910

astropy.io.ascii.write ignores fill_values
Update CHANGES.rst
Update CHANGES.rst

Update some of the descriptions to only use plain markdown instead of sphinx extensions.
Merge pull request #1140 from astrofrog/tests/pep8

PEP8 fixes in astropy/tests
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