afw table and record should have useful str() and repr()

To see the contents of an afw table in python, you have to do something like

catalog[0].extract("*")

which is totally not discoverable and returns a dict which doesn't print well. Much more useful would be for str() on a record to produce a pretty-printed list of the contents, and str() on a table to produce some nicely formatted summary (like a numpy recarray does, only printing a few things separated "..." for large tables).

I'm not sure what the best repr() output would be (certainly for a Record it should be a full dump of the contents), but currently table.repr()==table.str(), which is equally unhelpful.

Maybe this will come "for free" when we get astropy.table views, but it makes exploring the contents of a table a pain right now.