Utility to get documentation page from a live object

This is a little utility I cooked up a couple months back but forgot to PR.  It lets you find the URL to the doc page of any astropy object just by doing ``find_api_page(ICRSCoordinates)`` or whatever.  

I had hoped to have this be something we could integrate into ipython - e.g. type "ICRSCoordinates???" and it would open up the API page.  But the "?" syntax doesn't seem to be exposed right now in the Ipython API.  But this may be useful later if we want to attach URLs to docstrings or something.

It's still useful right now, too, it's just that I'm not sure that many people are likely to stumble across it.

@astrofrog @iguananaut - should this be in 0.2 (or in here at all, for that matter)?