Make Astropy logger 'opt-in'

As discussed on the dev mailing list:

https://groups.google.com/forum/#!topic/astropy-dev/N8r8JluJT0I

It might be a good idea to make the advanced features of the logger 'opt-in'. Specifically, this means:

* no overriding of ``sys.excepthook`` by default
* no overriding of warnings hook by default
* no logging to file by default

This requires a little thought, because we still want to have DEBUG and INFO messages go to standard output by default, and it would be nice to still be able to have warnings have pretty formatting like they do at the moment. This would require switching to ``log.warn``, but we didn't do this originally because ``log.warn`` doesn't have certain features of ``warnings.warn``.

cc @demitri @mdboom @embray