Small enhancements for io.ascii

Hi,
I started some months ago to work on `io.ascii`, to try to find ways to improve the memory consumption. I tried several things, made some [commits](https://github.com/saimn/astropy/commits/ascii_memory) and then I forgot this until now :/

So I have taken the few commits that can be interesting, rebased them (a lot of things changed in 3 months !), and this is it: nothing very fancy, just some cleanup while reading and testing the code. 

Actually the most interesting thing I tried (https://github.com/saimn/astropy/commit/894e007ad92c2636935d374b8697e8bc411d323c) has been done later in #2812 (with much more work) :-)