posse-post-discovery: more aggressively canonicalize urls

remove call to follow_redirects in refetch loop.
converts the following:

* twitter -> https://twitter.com/... (no http and no www)
* facebook -> https://facebook.com/...
* instagram -> http://instagram.com/...
* google+ -> https://plus.google.com/...

ongoing work on #171
minor: mind adding schem to the Args section of the docstring?