add a way for io.ascii set how masked values appear on output
Define our own masked class
add documentation for masked in io.ascii and for fill_values in general

closes #2255
closes #2314
fix line continuation in doctest
changes suggested in dicussion, pep8
make sure fill_values is always instance attribute, never class attribute
typo in documentation
add test for fill_values=None to restart Travis
typos in docs corrected
Merge pull request #2424 from hamogu/fillmasked

Add a way for io.ascii to set how masked values appear on output