Make a configuration item to control auto-logging

[mwcraig]

This would be very convenient who want logging to happen only when they explicitly put it in.

[mwcraig]

The astropy docs at http://astropy.readthedocs.org/en/stable/config/index.html#adding-new-configuration-items would be the place to start on this.

[janga1997]

@mwcraig I think I will take this up too. Will let you know of any doubts.