Update docs on reading CSV files.
This does the essential reworking of the documentation for `read_csv` and `read_table` for #227. I'm happy to do more doc updates.

I also think the autosummary API documentation isn't particularly good - it doesn't seem to handle classes at all. I'll see if I can copy what IPython uses (example output is [here](http://ipython.org/ipython-doc/stable/api/generated/IPython.testing.tools.html#tempfilemixin)).
